Nanocomposites in Dielectrics

5 Nanocomposites in Dielectrics Reza Taherian Faculty of Chemical & Materials Engineering, Shahrood University of Technology, Shahrood, Iran

5.1 Nanocomposites in Dielectrics In spite of several advantages of composite dielectrics, they include some disadvantages such as vulnerable to erosion and tracking, low resistance to surface pollution and acid, degradation under heat radiation, and early failure should be resolved. Nanocomposites can be considered as a solution for these problems. In these composites, polymer is matrix material with the addition of macro or nanosized filler [1
5].

5.1.1

Nanotechnology

The universal definition of nano for materials is that the particles contain at least one dimension between 1 nm and 100 nm. The nanodimension fillers can be categorized in three categories: One-dimensional (nanowire), two-dimensional (nanolayer or nanosheet), and three-dimensional (nanoparticles, nanotubes) fillers. Nanowire such as nanofiber; nanosheet such as graphene layer, expanded graphite, or nanocoatings; nanoparticles/nanotubes such as fluerene and carbon nanotubes (CNTs). Nelson has categorized the fillers as on the aspect ratio (the ratio of length to diameter) as it follows: Quasi-spherical particles, whiskers and rod particles, and platelet (lamellar) particles, that’s respectively the aspect ratio increases [6]. In another definition, the dimension is not limited to 100 nm, but also the nanosize is attributed to the size that more than that of the properties significantly change. Based on this definition, the dimension can be more than 100 nm. Nanosize the dimensions increase the effective surface between matrix and filler, thereby, changing significantly the properties. That is as a result of this fact in nanosized filler in a less volume percent loading of the filler the composite can achieve equal property in comparison to microsized filler. The more distribution and dispersion of nanofillers and the less filler loading of nanofillers are the most important advantage of nanosized fillers in comparison to microfillers. Electrical Conductivity in Polymer-Based Composites: Experiments, Modelling and Applications. DOI: https://doi.org/10.1016/B978-0-12-812541-0.00005-7 © 2019 Elsevier Inc. All rights reserved.

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The Role of Interface in Nanosize Dimension

3.0

100

75

2.0

50 1.0 25

Interface area (%)

Surface area/volume (nm–1)

Fig. 5.1 related the radius of particle to the surface area and suggested that when the size of the fillers is reduced, the specific surface area becomes very large. In other words, when the particle size decreases, the surface-to-volume ratio of the fillers increases, thereby the interface area in the composite between filler and polymer matrix increases. This considerably alter the nanocomposite properties [2]. Kickelbick [7] for the first one by a simple example shows the effect of breaking up the particles on the surface area and increasing the total number of atoms (N) and number of surface atoms (n). He supposed a cube composed by 16 3 16 3 16 atoms. In this cube, 4096 atoms which 1352 of them are located on the surface. This indicates that B33% of total atoms are on the surface. In the next step, the big cubic (or micrometer filler) is divided to 8 cubes 8 3 8 3 8 containing totally 2368 atoms on the surface that equals B58% of total atoms. If the dividing step continues on the same manner, the surface atoms achieve B88% of total atoms. When the size of particles is about 10 nm, nearly every particle contains one atom that equals a surface atom. This process mathematically shows the effect of refining the particles on the surface area value, thereby the nanocopmposite properties [8]. Fig. 5.2 shows that in one constant filler loading value, when the particle size is reduced, the surface-to-volume of fillers increases. In addition to this, it can be deduced from this curve that in a constant ratio of surface-to-volume, the needed filler value for the nanofiller laoding is much lower than that of

0.0 1

10

100

1000

Radius of the particle, (nm) Figure 5.1 The relation of surface area per unit volume or interface area via radius of the particle [1].

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Figure 5.2 Surface-to-volume ratios of nanocomposites as a function of nanoparticle loading [6].

microfiller loading. In other words, the percolation threshold that will be discussed in section 2, decreases by decreasing the particle diameter [9,10]. Polymer nanocomposites can be described as the composites in which small quantities of nanosized fillers are homogenously distributed in the polymer matrix by different weight percentages. The amount of nanofillers accumulated to the matrix are very diminutive (,10 wt.%). Opposite to this, the quantity of microfillers in polymer microcomposites is so high that it may reach up to 50% of the weight or higher of the total mass. The high filler loading in the microfiller composites makes the process ability of the polymer composite complex due to substantial rise in the viscosity and mixing the filler and matrix will be difficult. This leads to creating some agglomeration during the process and decreasing the homogeneity in distribution and dispersion of fillers in the matrix system. Therefore, the nanocomposites in lower filler loading contain a lower viscosity and processability in comparison to microcomposites [11]. However, the agglomeration phenomenon is more common for nanofillers due to electrostatic forces among the fillers that reduce uniformity the distribution and dispersion of the fillers. In some properties, such as electrical/thermal conductivity, nanocomposites have a significantly lower percolation threshold than that of microfiller composites. However, the agglomeration among the nanofillers increases the

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percolation threshold. In some cases, the local heating by heat treatment or passing the electricity across the composite leads to secondary agglomeration of nanofillers [9,10,12
14]. In the quiescent melt (due to absence of shear), a secondary agglomeration (or cluster formation) of fillers and formation of a conductive network of interconnected agglomerates could be formed, which leads to an increase of the conductivity of the composite melt. But also, there are a lot of variety in the shape of mixer blades. This shape can strongly the shear forces, thereby influencing the agglomeration type. Shear flow from one side, accelerate agglomeration and cluster formation and from other side, can destroy agglomerates, because the secondary agglomerates are more loosly packed than initial agglomerates. Therefore, it can be understood that there are a competition between build-up and destruction of secondary agglomerates (or physical clusters). The aspect ratio of fillers indirectly influences on agglomeration conditions. Therefore, the aspect ratio of fillers, surface energies of filler and matrix, mixing method, and blade shape are the parameters influencing on agglomeration status, thereby affecting electrical resistivity of composite [9]. The anealing and joule heating are two methods for heating the manufactured composite and probably formation of secondary agglomeration, thereby increasing electrical conductivity. Here, the time of heating is effective too. Secondary agglomeration in the most times improves the formation of conductive network, thereby decreasing percolation threshold. The best state of dispersion and distribution have been discussed in literature [26]. In order to enhancing electrical conductivity, it is better to fillers form a good dispersion and bad distribution. The configuration strongly depends on the surface energy of filler and polymer and mixing method [9,13
19]. The distance between adjacent fillers is greatly smaller in nanocomposites as compared to microcomposites in a similar filler loading. This is as a result of this fact the higher distribution and higher dispersion of nanofillers in the composite in comparison to microfillers. In addition, in nanofillers, specific surface area is approximately three times larger than that of microcomposites. Therefore, there is a lot of interaction of polymers matrices with fillers [11]. Nanocomposites are known to improve dielectric properties, such as the increase in dielectric breakdown strength, increase the permittivity (depends on the type of the nano fillers), suppressing the partial discharge (PD) as well as space charge, increase the resistance to surface discharge and prolonging the treeing, etc. [1,5]. However, the better

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dielectric properties in nanocomposite strongly depend on mixing method and the dispersion and distribution homogeneity. In a high agglomerated nanofillers in the polymer matrix cannot expect of improving properties.

5.2 Application of Nanocomposites in Dielectrics Lewis [3] is the first researcher that in 1994 introduced the nanocomposite for nanodielectrics to be used for power system insulating material. He believed that the use of nanocomposite could lead to another revolution in the dielectric material. There are many researches have focused on the improvement of the electrical properties of nanocomposite. Nanocomposite produces higher breakdown strength, higher permittivity, and higher dielectric constant than conventional composite. The composites contain three parts: Fillers, matrix, and interface. Below about the fillers and polymers used for dielectric industrial will briefly be discussed.

5.3 Matrix and Fillers in Dielectrics 5.3.1

Matrix

The polymers used for dielectrics include the thermoplasts, thermosets, and elastomers.

5.3.2

Thermoplasts

Thermoplasts when for the first time is heated can be softened and arrive to Tglassing and in more heating arrive to Tmelting, but this process is not reversible. Thermoplastic polymers become moldable above a specific temperature and solidify upon cooling that this process is reversible. The polymerization mechanism is usually addition polymerization (such as polyethylene (PE)) and condensation (such as nylon). Typical examples of thermoplasts are PE, PP, polyvinyl chloride (PVC), linear polyester, and polyamides (PAs). Almost 85% from the producted polymer in the world are thermoplastics and they can be divided into two large classes: Amorphous and crystalline. The

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crystallinity in polymers differs from metals. Here, when some alignment (may be local alignment) occur along some polymer chains, creates crystallinity that change properties especial optical properties. Crystalline regions are usually opaque (because it distorts the rays in a certain direction) that is not good for transparent polymers. In addition, the crystallinity of polymer influences on the mechanical properties. There is a degree of crystallinity in each polymer. Production condition such as additives, cooling rate, etc. can change the crystallinity. PEs, isotactic polypropylene (i-PP), polystyrene (PS) and PVC, represents over 70% of the total production of thermoplastics in the world. Polymers such as poly acetals, polycarbonate (PC), polyesters, polyphenylene oxide (PPO), blends and specialty polymers such as liquidcrystal polymers, PEEK, polyimide (PI), and fluoropolymers are increasingly used in high-performance applications [8]. 5.3.2.1

Thermosets

In the thermosets, usually three components should be present to polymerization occur. These are resins containing monomer, hardener, and stimulator. The hardener contains groups to react with monomer and creates crosslinks, thereby curing the polymer. The stimulator is optional and is for increasing the rate of the curing reaction. The curing process in thermosets is not reversible. The crosslinks in the thermoset polymers usually provide a ncomplex network for thermosets while in thermoplasts usually the linear chains are formed [8]. Curing can be performed either by heating generally, above 200 C or by high energy irradiation. Examples of this type of polymers are epoxy resins, polyester resins, polyurethane, phenolic resins. Fillers or fibrous reinforcements are often applied to enhance properties such as electrical/thermal, mechanical, and dimensional stability of thermosetting resins. Due to their excessive brittleness, many thermosetting polymers could be useless if they are not combined with fillers and reinforcing fibers such as fiber glass, fiber carbon, kevlar [8]. One of the most advantages of thermosets is the liquid state of these polymers that contains lower viscosity than that of thermoplasts. The lower viscosity is important in manufacturing the composites because eases the mixing process and permit higher filler loading. While in thermoplast often the viscosity decreases by heating the polymer more than Tm that this process influences smaller effect on viscosity, moreover, from the viewpoint of environmentally and energy consuming this process is not suitable.

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5.3.2.2 Elastomers Elastomers have a structure similar to thermosets with lower degree of crosslinks. Elastomers are elastic materials that can deform when force is applied and revert to the original shape when the force is released, such as rubber. Crosslinking increases the rigidity of polymer and decreases elasticity. Elastomers are flexible polymers that comprise a low crosslink density and generally have low Young’s modulus and high failure strain compared to other types of polymers. There are two main categories of elastomers: Elastomer with C 5 C double bonds in their polymer structure (i.e., styrene-butadiene copolymers, polybutadiene) and elastomers containing only saturated C
C bonds in their structure (ethylene vinyl acetate (EVA), ethylene propylene diene rubber) [22]. The most common polymers used for dielectric industrials are PE (for power cables), epoxy resins (insulators), polyester, silicone and imide (for electric machines, dry transformers), and silicone rubbers (for housings of power insulators). Low, medium, and high density of PEs have good mechanical, dielectric, rheological, corrosion resistance properties. However, they have low operating temperatures (below 90 due to low glassing temperature. This defect can be improved by addition of inorganic additives. Thermosetting resins have higher service temperatures because the curing reaction is not reversible (155 C-epoxy resins, 175 C-polyester resins, 180 C- phenolic resin, 200 C-silicone resins, and 240 C-imide resins) [8]. In high-voltage application, all three types of polymers (thermoplast, thermoset, and elastomers) are used.

5.3.3

Nanofillers for Nanocomposite Dielectrics

Polymer-based composites contain a mixture of two or more components, with two or more phases; the polymer as matrix and other components as fillers or reinforcement. The fillers may have different geometries, such as fibrous (continuous, short, or whisker), irregular flakes (carbon black), spheres, acicular and plate-like in shape (graphite), tubular (CNT), warm-shape (expanded graphite). The microfillers usually are used in a reasonable large volume concentration in polymers ( . 5 vol%). This volume concentration of fillers decreases for nanofillers (usually between 0.01 to 5 vol.%). The fillers can be continuous (or high aspect ratio), such as long fibers embedded in the polymer in regular arrangements (random distribution) extended across the microcomposite dimensions or discontinuous (or low aspect ratio), such as short fibers, whiskers, flakes, platelets, or irregularly shaped fillers (,3 cm

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in length) arranged in the polymer in different and multiple geometric patterns forming a microcomposite [8]. Fillers differ from one another in different ways: 1. Chemical compositions: Fillers depending the purity and their structure (crystalline or amorphous) have different electrical conductivity. From the viewpoint of chemical aspects, fillers can be classified in inorganic (i.e., oxides, hydroxides, salts, silicates, and metals) and organic (i.e., carbon, graphite, natural polymers such as polypropylen fibers and synthetic polymers such as kevlar fiber) substances. In different grouping fillers can be divided into natural and synthetic fibers. Table 5.1 shows that the fillers are natural (i.e., mineral, such as asbestos and animal, such as silk, wool, cellulose, etc.) and synthetic (i.e., organic fibers including Kevlar, carbon fiber (CF), fiber glass, Table 5.1 Examples of Different Types of Fillers

Origin

Chemical Structure Examples

Natural

Animal Mineral Cellulose

Synthesis Inorganic

Organic

BN: boron nitride.

Adapted table from [8].

Silk, Wool, Hair Asbestos Wood, Seed, Leaf, Fruit, Stalk, Bast, Grass Oxides TiO2, SiO2, Al2O3, ZnO, MgO, Sb2 O3 Hydroxides Al(OH)3, Mg(OH)2 Metals Al, Au, Ag, B, Sn, Cu, Steel Salts CaCO3, BaSO4, CaSO4, etc. Carbides silicates asbestos, talc, mica, nanoclay, kaolin Nitrides AlN, BN, SiC Carbon family Carbon and graphite fibers and flakes, carbon nanotubes, carbon black, graphene, graphene oxide Natural polymers cellulose and wood fibers, cotton, flax, starch Synthetic aramid, polyester, polyamide, polymers polyvinyl alcohol fiber

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carbon black, expanded graphite, graphene as well as inorganic fibers such as oxides and hydroxides: TiO2, SiO2, Al2O3, aluminum trihydroxide [Al(OH)3], magnesium hydroxide [Mg(OH)2], antimony trioxide (Sb2O3), etc.) [8]. Nanofillers can be divided into several categories depending to their shape: nanoparticle, nanofiber, nanotube, nanosheet, nanoplate, nanowire, nanorod, etc. [8]. Various nanoparticles, such as nanoclays (organomodified montmorillonite, etc.), nanooxides (TiO2, SiO2, Al2 O3, etc.), CNTs, nanosheet expanded graphite, carbon black (irregular), metallic nanoparticles (Al, Fe, Ag, and Au, etc.), semiconducting particles (SiC, ZnO, etc.) have been homogeneously dispersed in polymers, as provided in Table 5.1 [8]. 2. Shape and form: Shape and morphology of filler may be tubular, rod-like, particular, etc., that strongly influences on the interface properties. 3. Sizes: Size of filler may be micro or nano that can affect on the dispersion, distribution, percolation threshold, needed filler loading, etc. Reducing the particle size has been noted to reduce the percolation threshold, even with spherical particles [17,23,24]. 4. Aspect ratio: Aspect ratio of filler can considerably influence on the properties such as electrical conductivity due to easier forming a conductive network of fillers. The increase of the filler aspect ratio makes it easier to form a conductive network, thereby decreasing percolation threshold, but there is a technological problem: increasing the filler length, increasing the agglomeration. This limits increasing the aspect ratio in order to increase the electrical conductivity. But also, the changing mixing method or manufacturing may change this limit. For example, in hand lay-up method, the allowable filler loading is more than that of compression or injection molding [25]. The effect of aspect ratio is specially more evident in the low filler loadings [26]. Taipalus [27] reported that the increase of CF length causes to decrease the percolation threshold and increase the maximum electrical conductivity due to better forming the conductive network. Tsotra [28] has also reported the same results for epoxy polymer/ CF composite in different lengths of CF. Sohi et al. [29]

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resulted that the percolation threshold of composites consists of ethylene venyle acetate copolymer and CB, short CF and MWNT (multiwall CNT) have been achieved at 30, 15, and 5 wt.%, respectively. It would be related to the increase of the aspect ratio from CB, CF to MWCNT leads to increasing the electrical conductivity of percolation threshold from P/CB (2.5 3 1026 S/cm), P/CF (5 3 1025 S/cm) to P/MWCNT (2.5 3 1024 S/cm), respectively. In addition, it has been reported that the curve slope of electrical conductivity versus filler percentage from CB, CF to MWCNT has been increased [29]. The similar results about CB, CF, and MWCNT composite with poly methyl methaacrylate (PMMA) polymer have been reported by Buys et al. [30]; the greater the surfaceto-volume ratio of the particular and fibrous fillers, the more likely is inter-particle contact [25]. The more interparticle contact leads to lower interfacial contact resistance, thereby the higher electrical conductivity in fibrous composites compared to P/CB. Increasing aspect ratio of fibers also have a result similar to surface-to-volumer ratio. Jana [26] reported that the increasing aspect ratio in CF leads to increasing the mobile carrier concentration and increasing mean free path, thereby increasing electrical conductivity. In other words, by increasing aspect ratio “mean free path” increases leading to the decrease of percolation threshold. In the case of CB, it was also reported that the increasing aspect ratio of CB leads to decreasing percolation threshold and increasing the slope of conductivity/filler curve in P/CB in respect to P/G composite [31]. In the case of CNT, Carter [32], and Clingerman [33] also reported that the increase of aspect ratio in MWCNT resulted in decreasing percolation threshold and increasing the maximum of electrical conductivity of composite. The aspect ratio of fillers is not usually the effective value as high as the real value. Because, the bending and entanglement of fillers decrease the influence of aspect ratio. In the case of dielectrics, the aspect ratio can affect on treeing mechanism in dielectrics. 5. Surface energy: Surface energies of the polymer and filler determine the wettability value of the fillers by polymer. The wettability can affect the filler agglomeration, filler

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distribution, filler dispersion, composite porosity, thereby affecting the electrical conductivity of composite [34
36]. When the wettability is high value, the polymer surrounds each filler and inhibits the direct contact between fillers, decrease mean free path, thereby decreasing the electrical conductivity of composite. In addition, the low wettability leads to increasing agglomeration [37]. It has been established that partially agglomeration needs to perform conductive network, thereby increasing electrical conductivity [26]. This filler partiial agglomeration of fillers within the matrix improve the dispersion and distribution of fillers, thereby increasing electrical conductivity of composite. Because the better conductive network is formed in a weak distribution and good dispersion state [26]. This agglomeration should be so that some fillers connect together to shape conductive network, not leads to attachment a lot of fillers together in a specific regions without connecting these regions together. As it was previously mentioned post annealing and joule heating processes could lead to producing the secondary agglomeration, thereby increasing electrical conductivity. The equations to calculate the surface energy are as follows [37]: γ 2 γ SL cosðθÞ 5 S (5.1) γL γ SL 5 γ S 1 γL 2 2ðγS :γL Þ0:5

(5.2)

γS, γL, γSL, and θ are the surface energies of filler, polymer, filler/polymer, and wetting angle. The symbol of polymer wettability is cos(θ). The better wetting of fillers by the matrix is provided by the fillers containing low surface energy and the matrix containing high surface energy. However, the high filler wetting does not indicate to better performing conductive network. In order to achieving a better conductive network, partial agglomeration usually needs. As Eq. 5.1, the smaller differences between the two surface energies of filler and polymer result in the better wetting of the fillers sorounded by polymer and the lower wetting angle. This indicates that some difference between the surface energies of the filler and polymer is desirable [38]. Very small dimensions and large specific area of nanofillers (fillers containing one dimention below 100 nm) in polymer nanocomposites

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provide different physical and chemical properties compared to traditional composites. Selection of the nanoparticles for a proper commertial application depends on the desired value of electrical, mechanical, and thermal properties as well as the cost. For example, CNTs improve the electrical and thermal resistivity, CF improves electrical and mechanical properties, Al2O3 is usually selected for high thermal conductivity, and TiO2 nanoparticles (anastase or rutile) have photocatalytic properties. Calcium carbonate (CaCO3 ) typically acts as a low cost filler materials and is used for high number of deposits. SiC is applied for mechanical strength, hardness, corrosion, and nonlinear electrical behavior. SnO2 and AgO have weak electrical conductivity in the range of semiconductors. Due to its high thermal conductivity and nonlinear electrical characteristics, ZnO is employed in composites for electric stress control [8]. Regarding to high surface area in naofillers, they can be used in significantly lower filler loading even 0.01 vol.% for improving or adjusting different properties, such as electrical, mechanical, thermal, and optical or fire-retardancy of nanocomposites, provided that the disentangling process of fillers is effectively performed and the nanoparticles properly distributed through the polymer, because the agglomeration among the fillers is common due to van der waals attraction. The good dispersion of the nanofillers into the polymer matrix can be achieved by using different methods and preparation techniques including ultrasonic vibration or special sol
gel techniques, high shear energy dispersion mixing, and/or through a surface modification or functionalization of the nanofillers [8]. Table 5.2 describe the types of functionalization of CNTs and their effectiveness on deagglomeration of CNTs in the matrix. 5.3.3.1

Common Nanofillers

Nanosheet-clay: Nanoclay (Fig. 5.3A) is one of the nanofillers which is commonly used in HV insulators. This filler is a layered silicate/clay minerals categorized in the class of silicate minerals and phyllosilicates. These minerals include synthetic and natural clays such as bentonite, mica, fluorohectorite, laponite, magadiite. Layered silicate structure can be illustrated as 2D layer of two fused silicate tetrahedral sheet with an edge shared octahedral sheet of metal atoms, like Mg or Al [39]. A silicate is a material containing prevailingly Si and O (base formula SiO4) forming a tetrahedral structure with several allotropes. Since each tetrahedral has an excess of negative net electrical charge, therefore, silicate

Table 5.2 Advantages and Disadvantages of Various CNT Functionalization Methods [62]

Method

Principle

Chemical methods

Side wall

Physical methods

Defect Polymer wrapping Surfactant Adsorption Endohedral method

Hybridization of C atoms from sp2 to sp3 Defect transformation van der Waals force, π-π stacking Physical adsorption Capillary effect

Possible Damage to CNTs

Interaction Easy With to Polymer Use Matrix

ReAgglomeration of CNTs in Matrix

ü

✕

s

ü

ü ✕

ü ü

s v

ü ✕

✕ ✕

ü ✕

w w

✕ ü

S: strong; w- weak; v- variable according to the miscibility between matrix and polymer on CNT.

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should bond to some metal cations to neutralize these negative charges. These metal ions usually include of Fe, Mg, K, Na, and Ca. In phyllosilicates, a central octahedral sheet of alumina is fused to two external silica tetrahedrons [6]. TiO2: The nanoparticles of TiO2 (Fig. 5.3B) usually are used in order to increase UV resistivity and hydrophobicity properties in the composites. Fothergill et al. [39] used 23 nm titanium dioxide nanoparticles to incorporate into an epoxy matrix to form a nanocomposite structure. The nanometric particles can results in a significant change of composite properties, traced to the migration of the internal charge as compared with microsized TiO2 particles. It was appeared that, when the size of

Figure 5.3 The images of different fillers used for dielectrics: (A) nanosheet-clay [1], (B) TiO2 [2,3], (C) MgO [4], (D) Mica [7,8], (E) Al2O3 [9,10], (F) SiO2 [11,40
50].

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Figure 5.3 (Continued).

the inclusions changes from micro to nanosize, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is guessed that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through the nanocomposite that is a suitable property for nanofilled dielectrics [20]. MgO: MgO (Fig. 5.3C) is a ionic insulator with conduction band is found to be 4.6 eV [40]. However, it has high tendency to absorb water. When dissolved in water or molten NaCl will conduct electricity. Magnesium oxide stays solid at such high temperatures. Therefore, it

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remains nonconductive. It is used for high-temperature electrical insulation. ZnO: Zinc oxide is an inorganic compound and is powder shape. This material is widely used as an additive inside numerous materials including plastics, ceramics, glass, rubber (e.g., car tyres), batteries, fire retardants, etc [6]. The electrical conductivity, structural, and optical properties of ZnO nanostructured semiconductor thin film prepared by sol
gel spin coating method and Pulsed Laser Deposition have been investigated [41]. The electrical conductivity of the film depends on temperature that changes dominant conductivity mechanism. Mica: From the viewpoint of composition, mica is a mixture of silicate of aluminum with traces of other oxides. The most common types of mica are muscovite (K2O-3Al2O3-6SiO2-2H2O) and phlogopite (K2O-7MgO-Al2O3-6SiO2-3H2O) (Fig. 5.3D). Mica has a complex structure consists of silicon atom layers in centers of tetrahedrons formed by oxygen atoms. Potassium atoms and hydroxyl groups provide the connections between layers. This structure leads to flaky-like structure of mica can be split into the thin layers and absorbs the water droplets into these layers [10]. Mica has a high thermal endurance and starts to lose its water at a temperature of 500 C, although some types endure even above 1100 C. Regarding to the highest permitted temperatures for electrical machines that is usually about 200 C, mica can be suitable dielectric. Both properties of dielectric strength and surface resistance of mica are high while the dielectric losses are low. One of the best specifications of mica is the higher resistance to creepage currents and PDs in comparison to the best organic insulators. Regarding to earlier mentioned, mica is almost one of the most required material in high-voltage electrical machines [8]. Nowadays mica is applied for the manufacture of composite materials based on mixing with natural (shellac, bitumen, etc.) and synthetic (bakelite, epoxy, polyester, etc.) resins to be used for the insulation systems of medium and high power electrical machines. Nowadays, mica is used mainly for production of mica paper composing of extremely small flakes of mica. This composite is produced in the same way as paper prepared by using VPI (vacuum pressure impregnation) process and this mica paper is employed in ground wall insulation of turbine generator stator coils [8]. Al2O3: Kadhim et al. [42] in the Epoxy /Al2O3 nanocomposites found out that the relative permittivity has an essential decrease even in 1 wt.% Alumina as compared to unfilled epoxy. It is observed that

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the relative permittivity at 2, 3, and 4 wt.% of Al2O3 nanoparticle increased with increasing weight percentage of Al2O3 nanoparticles, respectively. At 5 and 7 wt.% relative permittivity decreased again. Tanδ decrease with increasing particle weight percentage of Al2O3 until reached 5wt.%, tanδ begin to increase by increasing alumina percentage but still less than of unfilled epoxy. This property has been attributed to the interaction between epoxy chains and the nanoparticles [42] (Fig. 5.3E). SiO2: Roy et al. [39] reported that the incorporation of silica nanoparticles into polymer observably increased the breakdown strength and voltage endurance compared to the incorporation of micron scale filler materials. In addition, nanofillers decease the dielectric permittivity in nanocomposite as compared to the base polymer. It can be related to the more interfacial contact resistance between nanofiller and polymer in comparison to that of microfillers. Silica nanoparticles (Fig. 5.3F) are the common used nanofiller for polymeric materials because of the high surface area of silica nanoparticles, both in the state of fumed and precipitated, the surface silanol functional groups play a key role in determining the particle physical properties especially these silanol groups impart a hydrophilic character to the material. Silanol is the most common interfacial materials between filler and matrix. The surface hydroxyls create the weak hydrogen bonding between nanoparticles, thereby agglomeration phenomenon that is a common phenomenon during the compounding process where powerful shear stresses are applied on the surface of the agglomerates. The agglomeration possibility increases by decreasing particle size especially in nanoparticles. In order to effective dispersion of these nanofillers in the matrix, usually a surface modification (functionalization) is performed on the nanosilica by coupling agents like trialkoxysilanes as silanization process. This process converts silanol groups into hydrophobic alkyl groups through self-assembly with silane-like molecules. In addition to silica, many materials such as mica, glass, and metal oxide surfaces due to containing hydroxyl groups can be silanized, and displace the alkoxy groups on the silane thus forming a covalent Si-O-Si bond. In addition to trialkoxysilanes, other compounds such as alkylchlorosilanes and alkylsilazanes can be used to modify the filler surface due to their ability to hydrolysis and condensation reactions as observed in Fig. 5.4 [6]. Functionalization process on silica plays an important role in lowering the agglomeration phenomenon, thereby effective dispersion and distribution of silica in the matrix [6].

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Figure 5.4 Schematic representation of silanization reactions [6].

5.4 Mixing Methods of Fillers and Polymers in Nanocomposites 5.4.1 Mixing Methods Mixing method is one of the most effective parameters influencing the alignment, distribution, and dispersion of fillers in the matrix, thereby the composite conductivity. During mixing, the value of shear rate, viscosity of the polymer melt, and duration time are the deteriminative parameters in dispersion and distribution of fillers in the matrix. The agglomerates break under shear or tensile stress if the applied stress is greater than the agglomerate strength or hydrogen bonding between particles [13,54]. Shear stress is directly related to viscosity and shear rate, while tensile stress is directly related to extensional viscosity and tensile strain rate [17,55,56]. There are two categories for mixing of filler and polymer: Chemical and mechanical methods. Mechanical route: It includes high shearing techniques using mechanical forces such as mechanical stirring, sonication, microfluidizing, and calendaring [57]. These methods involve imposing high shear forces on the polymer/filler mixture or between fillers that leads to the homogeneous dispersion of the fillers. The size and shape of the propeller, mixing time, and the mixing speed control the resulting dispersion [57]. Chemical methods: The chemical strategies typically involve either a covalent modification of the surface of the filler by functionalization methods or the use of a dispersant or surfactant (steric or electrostatic

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surfactants), being polymeric or supramolecular in nature. The chemical strategies are more effective in preventing the re-agglomeration of the fillers together after the dispersion process has been carried out. In addition, in the case of fibrous fillers, the possibility of fiber breakage is more in mechanical routes as compared to chemical methods, but the mechanical routes are most environmentally friendly. It is more desirable to combine the mechanical techniques with chemical methods. Chemical strategies inhibit the four common instabilities (agglomeration, sedimentation, particle growth, and particle reaction) of the polymer/filler solution (especially in nanofillers) and prevent their reagglomeration, which ultimately leads to a better dispersion when coupled with the mechanical techniques. Furthermore, the chemical techniques leads to surface modification or functionalization, thereby interfactial bonding between filler and matrix [57]. The electrical properties of materials can be shown in these properties: Dielectric breakdown, tracking and erosion resistance, PD resistance, space charge behavior, permittivity and dissipation factor, and loss tanδ. The earlier mentioned properties are improved by using nanofillers instead of microfiller if the fillers to be effectively dispersed. The reason of this improvement is related to providing a much great interfacial area or large interaction zone between polymer and nanoparticle, changing the polymer morphology due to the surfaces of particles, reduction in the internal field caused by the decrease in size of the particles, changing the space charge distribution, and scattering mechanism [39]. The interfacial regions between filler and matrix have a different properties in respect to the bulk. They can have a different mobility than the bulk material, that results in an increase or decrease in glass transition temperature. 5.4.1.1 Processing Method of Polymer-Based Nanocomposites [21] 1. Intercalation method are as follows (Fig. 5.5): a. polymer or pre-polymer intercalation from solution b. in situ interactive polymerization c. melt intercalation 2. Sol
gel method 3. Molecular composite formation method: Liquid crystal polymer alloy formation method 4. Nanofiller direct dispersion method

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Figure 5.5 Preparation method of polymer-layered silicate nanocomposites and type of nanocomposite structure [1,2].

5.4.2 Intercalation Method Solution blending: This method is categorized to three steps. The first is based on a solvent system in which the polymer or pre-polymer is soluble and silicate layers are swellable. The solvent input the silcate layers and form a weak bond with each one. The layered silicate is swollen in a solvent, such as water, chloroform, or toluene. In the second step, the polymer and layered silicate solutions are mixed together, the polymerization is developed and the polymer chains intercalate and displace the solvent within the interlayer of the silicate. This mixing will be performed by high shear mixer. In the third step is the evaporation of solvent that the solution is heated to be dried and to create the intercalated structure of polymer layered silicate nanocomposite [21]. In situ blending: In this method, at first, the layered silicate is swollen within the liquid monomer or a monomer solution so that the polymerization may occur between the intercalated sheets. Polymerization can be initiated either by heat or by an organic initiator or catalyst fixed through cation exchange inside the interlayer before the swelling step. The advantage of this method is a properly dispersion and distribution of polymer chains or polymer grafting within intercalated sheets of filler such as expanded graphite [21].

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Melt blending: This method is usually used for thermoplastics because after melting of polymer, fillers are added into the melt. Here, the matrix is arrived to the temperature higher than melting point and then the fillers are added to melt and by a blender a high shear force is imparted into the fillers to disentangle the fillers. The effective mixing in this method is very important because in this method as compared to solution blending the viscosity is higher, therefore, in this method the agglomeration between nanofillers is more common in comparison to other methods. In addition, the allowed filler loading in this method is lower than solution blending because of lower viscosity (the fillers decrease the viscosity). This method has advantages such as the absence of organic solvents and more compatibility with current industrial process, such as extrusion and injection molding as compared to two previous methods, however this method has some limitations such as filler loading value and is limited to thermoplastic polymers [21]. The disadvantages of melt blending method: (1) difficulty in disentangling the nanofiller and the high probability of fragmenting and erosion the nanofibers, nanotubes, and other brittle filler against high shear force; (2) the high energy consumption to heat the polymer and in addition to environmental problems. The complexity and the tip form of blade in the blender has an essential effect on shear force value, thereby disentangling process and effective dispersion of nanofillers in the matrix. 5.4.2.1 Sol
Gel The sol
gel method is a kind of polymerization that the reaction is started from the precursor of metal alkoxide that resulted in metallic oxide such as SiO2 and TiO2. Its advantages are the low temperature process, simple process, high purity, high variety in the product (fiber, powder, coating, and bulk), availability of some alkoxides, etc. The process consists of these steps: Hydrolysis and condensation to achieve “sol” and polymerization to achieve “gel” and then drying and firing processes. Aerogel as high porous material, powder material, and high density material can be achieved by controlling the drying step. This step is very important because the solvent is evaporated and by rapid or slow drying is resulted in different materials. The sol
gel process is formation of a metal oxide involves connecting the metal centers with oxo t form (M-OM) or hydroxo (M-OH-M) bridges, and generating metal-oxo or metal-hydroxo polymers in solution [58].

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Figure 5.6 Sol
gel process for production of SiO2 [65].

This method has been utilized to fabricate glasses and ceramics specially oxides such as SiO2, TiO2, Al2O3. Recently, at the same time, it has been used for polycrystals, porous composites, and organic-inorganic composites. Sol
gel reaction is started from metal alkoxide precursor, with formula of (MOR)n. M is the metal, O is oxygen, and R is an alkyl group. Metal alkoxide should be homogeneously soluble and to be dispersed in water, alcohol, acid, or ammonia. Metal alkoxide is hydrolyzed through reaction with water and turns out to be metal hydroxide and alcohol. Some alkoxides are abundant and inexpensive such as Si, Na, Cu, Al, and Ti. However, some of them is uncommon and expensive such as alkoxide of Ba, Zr, Ge, V, W, and Y. The most common alkoxide is silicon alkoxides such as tetraethoxysilane (TEOS) or tetramethoxysilane (MTEOS). The hydrolysis and condensation or polymerization reactions in the case of TEOS are as follows (Fig. 5.6) [59]: SiðOC2 H5 Þ4 1 H2O- ðOC2 H5 Þ3 Si 2 OH 1 C2 H5 OH  Si 2 OH 1 HO 2 Si  -Si 2 O 2 Si  1 H2 O  Si 2 OH 1 ðOC2 H5 Þ3 Si 2 -  Si 2 O 2 Si  1 C2 H5 OH

(5.3) (5.4)

Depending on the amount of water and catalyst present, hydrolysis may proceed to completion to silica. In order to complete hydrolysis often requires to be used an excess of water and/or a hydrolysis catalyst such as acetic acid or hydrochloric acid [59,60]. After drying process, the liquid phase is removed from the gel. After that firing or calcination may be performed to favor further polycondensation, complete curing, and increasing the mechanical properties [58].

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5.5 Fillers Surface Treatment The most important challenge in mixing the polymer matrix and nanofillers is the homogeneous dispersion and distribution of the fillers in the matrix. Because due to van der Walls force and interfacial tension. Surface treatment such as physical and chemical functionalization as well as using surfactants are the best solution for this problem. This work in addition to avoid agglomeration inhibits the other instabilities in the nanoparticle-contained solution. The mechanism of surfactant was mentioned later. Here, functionalization is defined and is categorized [8].

5.5.1

Functionalization

Functionalization is the chemical and physical surface treatment on the nanofillers in order to disentangling of fillers and better contact of filler to polymer matrix. Therefore, functionalization via two things can alter the properties, strength, stiffness, permeability, optical clarity, and electrical conductivity of the nanocomposites consistently: (1) the homogeneously dispersion of the nanofillers throughout the matrix material; (2) the more effective interfacial bonding between the nanofillers and the polymer matrix material [8]. Functionalization can also significantly influence the dispersion of the nanofillers and induce some defects on the filler surface, thereby influencing on the electrical conductivity of composites. The functionalization of CNT leads to breakage of surface bonds and adhering the functional groups such as hydroxyl and carboxyl to these free bonds. This leads to better adhering the polymer chains to the CNT wall and increases the bond strength of polymer matrix and CNT walls, thereby increasing the mechanical strength of composite. However, the breakages of carbon-carbon bonds on the CNT surface consider as surface defects. Because decrease the mean free path of electrons and leads to scatter electrons thereby generally reducing the intrinsic electrical conductivity [57,61
63]. This reverse effect can also be attributed to the dependence of the percolation threshold on the aspect ratio of the nanofillers. Chemical and mechanical functionalization usually tend to reduce the overall aspect ratio of the carbon fillers such as CNTs, which is not desirable when aiming to increase electrical conductivity and reduce the percolation threshold. Furthermore, the functionalization creates some functional groups on the filler surface that facilitate reaction of the epoxy resin with the surface groups of nanotubes forms an

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electrically insulating epoxy layer, which increases the distance between individual tubes, thereby making harder the tunneling of electrons from tube to tube [8]. The functionalization is categorized into two types (Figs. 5.7 and 5.8) [64]: 1. Chemical functionalization (covalent functionalization): a. oxidation followed by Amidation or Esterification or Thiolation or Silanization, or Polymer grafting (direct side wall functionalization) b. fluorination and derivative reactions—cycloaddition (defect functionalization) c. plasma

Figure 5.7 Schematics of CNT functionalization using non-covalent methods (A) polymer wrapping; (B) surfactant adsorption; and (C) endohedral method [1,64].

Figure 5.8 Strategies for covalent functionalization of CNTs (A) direct sidewall functionalization; (B) defect functionalization [64].

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2. Physical functionalization (noncovalent functionalization): a. wrapping b. endohedral c. surfactant treatment The effect of functionalization on toughness and strength has been described in Fig. 5.9. Weak interfacial bonding is a result of a nonwetting phenomenon between the nanofiller and polymer matrix, which is caused by the lack of functional groups on the nanofiller. But by increasing the chemical bond (covalence bond) between nanofiller and matrix, the interfacial energy increases, thereby the stress for pulling out of the filler through the matrix is higher. This leads to increasing toughness and strength. Because the crack cross from longer pathway to cross the filler [6].

Figure 5.9 The schematic of pullout mechanism in the matrix, (A) the initial CNT, (B) pullout without any interface resistance, (C) CNT failure, (D) wall failure of CNT against stress, (E) properly interface resistance against pullout and bridging mechanism of CNT within the matrix [1]; (F) schematic of crack bridging and fiber pullout [2].

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5.5.2 The Effect Functionalization on Mechanical Properties Crack bridging (Fig. 5.9) is one of toughening mechanism for particle or fiber reinforced composite materials. In this mechanism, the crack bridges on the fiber and slows down the crack growth. As the stress increases, fiber pullout or fiber breakage will occur that leads to partial dissipation of strain energy near the crack tip. The pullout strength of composite depends on the filler length, surface asperity of fillers, and surface modification of fillers as well as functional groups common between filler and matrix [6]. Another mechanism in toughness is crack deflection. Here, crack deflect from the straight path. This increases the absorbed energy during crack growth. In this mechanism the interface bond between fibers and matrix should be weak to direct the crack through interface path. The researches show that microcracking close to the crack tip can reduce stress concentration, decrease the local modulus and the local stress intensity and improve the fracture toughness. Microcracking in composites is usually due to differential thermal shrinkage between polymer and fillers. But, this decreasing the crack development strongly depend on the certain size and spatial distributions of microcracks in the vicinity of the main crack tip [6]. In chemical functionalization, the surface of fillers may be damaged, therefore the electrical conductivity and thermal conductivity of fillers, thereby nanocomposite will decrease. However, in physical functionalization the filler surface does not change chemically, therefore the electrical/thermal conductivity does not change. But also these properties may strongly vary by dispersion and distribution of fillers by functionalization process. On the other hand, the mechanical properties will be improved by chemical functionalization as compared to physical functionalization, due to the chemical bonds between filler and matrix. In direct side wall functionalization, at first a process of hydrogenation, colorization, or fluorination is performed to create a free bond. Then, the appropriate functional groups attach to the nanofiller. In defect functionalization, at first the filler is oxidized in an oxidative solution such as nitric or sulfuric acid. Then, via a process such as sterification, thiolation, silanization, and polymer grafting the desired group is attached to the filler surface. Due to the covalent attachment of functional groups to the filler surface, the similar groups of monomer can react better with the filler surface and start to grafting. As a result, a stronger bonding between filler and polymer is formed [8].

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Plasma method is one technique to modify chemically and physically the surface of the inorganic nanoparticles, without influencing the bulk properties of the fillers. Plasma techniques increase the surface roughness of fillers and make the surface of the particles more wettable, harder, and more proper to adhere to the polymer chains [8]. In the presence of two or more monomers, graft copolymerization and polymerization reactions can be carried out by plasma treating that is accompanied with the preparation of fillers with controlled surface properties. The limitation of this technique is that the experimental conditions require a very expensive vacuum system. It has been established that surface modification of the inorganic particles by plasma or other high energy rays can produce excellent integration and surface activation, thereby good adhesion between the polymer matrix and filler [8]. Usually, for the chemical treatment of the filler surfaces, different types of silane coupling agents such as 3-aminopropyl triethoxysilane are used that can react with the hydroxyl groups of inorganic and organic groups of polymer matrix via condensation reaction. The change in the surface polarity in conjunction with steric hindrance mechanism, that is due to attachment of organic micells to particle surface, enables a better dispersion between the modified nanoparticles and polymer matrix. Another typical coupling agent, similar to the one is titanate coupling agent (i.e., tetra-isopropyl titanate), where the same effect is obtained [8].

5.6 Instabilities and Surfactants The large surface area of nanoparticles creates high total surface energy, thereby increasing the collisions between nanoparticles which is thermodynamically unfavorable. High surface energy of particles leads to tendency of agglomeration of particles to minimize the surface energy. Agglomeration can cause to occurring other instabilities including rapid settling/creaming, crystal growth, and inconsistent dosing in nanosuspensions. The best strategy to solve this problem is to introduce stabilizers to the solution. These stabilizers should be safe and they should properly wet the particle surface and offer a barrier to prevent nanoparticles from agglomeration [65]. Surfactants by three mechanisms usually inhibit the instabilities in both aqueous and nonaqueous medium colloidal suspensions: Steric, electrostatic, and electrosteric. Fig. 5.10 shows the instabilities and the mechanisms. In electrostatic repulsion, the nonionic stabilizers and for

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Figure 5.10 The different of instabilities are possible in a solution containing nanoparticles or nanosuspension [64].

steric repulsion, the ionic stabilizers are used. The electrostatic repulsion usually is used for aqueous suspension. The repulsive forces between particles are as a result of overlapping of electrical double layer (EDL) surrounding the particles in the solution that prevents the colloidal agglomeration. The EDL consists of two layers: (1) stern layer composed of counter ions attracted toward the particle surface to electrically neutralize the system and and (2) Gouy layer which is essentially a diffusion layer of ions (Fig. 5.11). As Fig. 5.11, in a solution containing nanoparticles four instabilities can be formed: Crystal growth, sedimentation, agglomeration, and chemical reaction [66].

5.6.1 Short-Chain Polymers With a Functional Head Group (Surfactants) Surfactants usually contain a short-chain organic tail (containing up to 50
100 carbon atoms) and a functional head group that is nonionic or ionic in nature named micell (Fig. 5.12). It can be stated that in nonionic surfactants, the head group may be polar but does not ionize to produce charged head groups. Nonionic surfactants are more commonly effective in organic solvents. Adsorption onto the particle surfaces may

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Figure 5.11 Illustration of classical DLVO theory. Attractive forces are dominant at very small and large distances, leading to primary and secondary minimum, while repulsive forces are prevailing at intermediate distances and create net repulsion between the dispersed particles, thus preventing particle agglomeration [64].

occur either by van der Waals attraction or, by stronger coordinate bonding based on Lewis-type chemical bond [66]. In the Lewis theory of acid-base reactions, bases provide pairs of electrons (such as N or O) and acids accept pairs of electrons (such as Al, Mg) and these electrons are shared between two atoms [66,67]. Positive-charged elements such as Al and Na act as Lewis acid and the negative-charged elements act as Lewis base. But also, the metallic ions such as Mg21, Al31, Ca21 act as electron acceptor against the ions with lower charge such as Na1 (Fig. 5.12A). In the case of cations, the more the ion charge and the smaller the ion size, the greater the ion’s ability to replace the sodium. In the case of Fig. 5.12B, it can be seen a or AlSiO2 negatively charged ions such as SiO42 4 4 can replace the 2 CH3COO . It can be seen about the anions, that the more anion charge and the bigger anion size, the greater the ion’s ability to replace the CH3COO2.

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Figure 5.12 Schematic of a steric repulsion between organic tails and organic micelles in (A) anionic surfactant and (B) cationic surfactant [65].

Stabilization most likely occurs by steric repulsion between the organic tails or micelles attaching on the particle surface (Fig. 5.12). Fish oil is a widely used dispersant for the particles of Al2O3, BaTiO3, and several other oxides. The oil fish consists of a mixture of several short-chain fatty acids with the alkyl chain containing some C 5 C double bonds and the functional end group being a carboxylic acid (COOH). Polyisobutylene succinamide, with commercial name of OLOA-1200, is commonly used for dispersing carbon allotropes (such as CB, graphite, graphen, etc.) in organic solvents [66]. Ionic surfactants are described as either anionic, that the functional head group ionizes to form a negatively charged species, or cationic, that the functional head group form a positively charged head group (Fig. 5.12). The ionic surfactants are effective in aqueous solvents. By dissociation of anionic surfactants usually form negatively charged

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oxygen species. Afterwards, by electrostatic attraction, these negatively charged species adsorb to the positively charged particle surfaces. By electrostatic repulsion between the negative charges due to the adsorbed surfactant molecules or micelles, stabilization of the suspension occurs (Fig. 5.12A). Cationic surfactants commonly consist of positively charged nitrogen species on dissociation that by electrostatic attraction, these positively charged species adsorb to the negatively charged particle surfaces (Fig. 5.12B) [66].

5.7 Interaction Zone Tanaka [21], has proposed a theory about interaction zone. They assumed the interfacial area between the filler and polymer matrix as interaction zone. They suggested this region consist of three layers [22]: 1. Bonded layer (first layer): A layer that is a transition layer, having a thickness of 1 nm, which tightly bonds both inorganic and organic substances by coupling agents such as silane. In general, this layer is bound by ionic, covalent, hydrogen, and van der Waals bondings, and the strength is in this order [6,22]. 2. Bound layer (second layer): A several nonmetric layer (2
9 nm) of polymer chains that are strongly bound, interacting with the first layer (bonded layer) and the surface of inorganic particles. Morphological structure is rather orderly in this region. The thickness value of this layer, depends on the strength of the polymer particle interaction so that the stronger the interaction, the larger the bound polymer fraction. Chain mobility and crystallinity are related to this layer. Chain mobility can alter with Tg of polymer. Chain crystallinity degree can alter by the type and the value of filler. The chain crystallinity includes the orientation of constituent radical groups and polymer chains stacked to the surface of nanofillers, as well as the folded structure of polymer chains. Crystallinity is also sensitive to interfacial interaction. Plasticizers in the shape of big organic molecules usually locate between the chains and decrease the crystallinity. Curing agents adsorbed selectively to nanofillers form a layer of stoichiometrically crosslinked thermoset

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region with excess curing agent. Such as the spherulite region around the nanoparticles in the case of PA/layered silicate nanocomposites that is named second layer [6,22]. 3. Loose layer (third layer): A layer that loosely interacts with the second layer and has a thickness of several tens of nm. The third layer is a region loosely coupled to the second layer. When in nanocomposite, curing agent attaches to the nanofiller surface, a crystallized layer saturated of the curing agent stoichiometric (bound layer) forms near the filler surface. However, the third layer will be somewhat depleted of curing agent and thus less than stoichiometric crosslinking will result. This layer a different chain conformation, chain mobility, and even free volume or crystallinity from the polymer matrix. This phenomenon has been observed in epoxy/titania nanocomposites [6,22]. 4. Double layer: An electric double layer overlapping the above three layers, there is an electric double layer which has a coulombic interaction that charges the nanoparticle positively or negatively [22]. The previous layers have the different chemistry on the nanofiller surface, however, double layer contains the charged ionic layer. When a polymer has mobile charge carriers, they are distributed at the interface of filler and polymer in such a way that the opposite charge carriers with the opposite polarity are diffused outward from the contact surface to the Debye shielding length. This layer corresponds to the Gouy-Chapman diffuse layer. The charge value decreases exponentially with distance of the particle surface. The Debye shielding length is approximated about 30 nm. The electric double layer forms a long distance dipole inhibit approaching the particles together. The dipole moment can affect electrical conduction and dielectric properties in the low frequency region. The high density polymers such as PE, PP, and EVA tend to become positively charged, while lower density of silicone elastomer, PA and epoxy tend to become negatively charged [6,22]. It should be mentioned that in induction of high voltage in dielectrics, the electrons should pass three layers and electric double layer to arrive to nanoparticle. When the filler is nanosize, the electrons with difficulty can pass through the barrier of these layers. Moreover, from the polymeric chains, there is not possibility for electron passage. Therefore, in composites containing the nanofiller, beak down is harder

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and the dielectric properties improve as compared to composites containing the microfillers [22]. Since the interaction zone for nanocomposite is far larger than for microcomposite, due to higher surface area to volume ratio in nanofillers, it has a great influence on the property improvement. Nanoparticles provide a superior interface region between polymer matrices, and thus a large volume of polymer belonging to the interfacial zone results in higher resistance against erosion. Normally, the degradation occurs in small isolated regions that form channels around existing nanoparticles, so good dispersion of nanoparticles will improve the resistance to degradation or erosion on the surface of the nanocomposite material [22,68,69]. Tanaka [21] guested that in the outer double layer, usually some defects and impurities are present. By considering these impurities as traps, it has hypothesized that traps are accountable for change in different dielectric properties such as space charge, that is, larger surface area of nanoparticles leads to change in density of traps, which reduces the space charge. In other words, the addition of nanoparticles leads to an increase in charge carrier mobility and decrease in average hopping distance relative to polymer matrix [5].

5.8 The Effect of Nanofillers on the Properties of Dielectrics 5.8.1

Permittivity

There is some reports on the increase of permittivity values by using nanofillers in the polymer matrix. This can be due to the higher permittivity of nanofiller than the base polymer, as well as due to overlapping of interaction zones. But also, the temperature and frequency can also influence on the permittivity values of nanocomposites [5,8]. If a high weight percentage of inorganic microfillers are entered into polymers, usually the relative permittivity of the composite increases. This is due to the higher permittivity of fillers as compared to the base polymers. Interface of fillers and matrix creates Maxwell-Wagner interfacial polarization. This type of polarization will increase the values of loss factor, too [8]. But in the nanofillers, there is an interaction zone, which surrounds the nanoparticles, has a profound effect on the dielectric behavior of nanocomposite and encourage to movement and reorienting the

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dipolars within interaction zone. This behavior could be also due to locking the epoxy molecules end-chains of side-chains by the presence of nanoparticles. However, this behavior of nanocomposites changes at low frequencies, when charge carriers have some restricted freedom of movement within the material and they may follow tortuous paths through the nanocomposite under the influence of the electric field, that do not allow complete transport through the material [8].

5.8.2 Electrical Conductivity Electrical field stress concentrations can be occurred when the electricity enters from one cable to another cable and this sudden change in material create electrical field stress concentration. This stress can be assumed a common problem in many high- and medium-voltage applications such as cables accessories, generator, or motor end windings or bushing. This electrical stress concentration can create breakdown, flash over, and corona phenomena between fittings. In order to solve the problem, it is necessary to control the electrical field throughout materials and the conductivity and nonlinearity on changing conductivity in the materials to be properly controlled. Using functional graded materials (FGM) to control grading the electrical field is one of the solutions. Using FGM avoids local surface stress at interfaces, in such away that, which will not exceed the breakdown strength in any location. FGM application in insulators is described in the following chapters. Other solution method is that the electrical field to be distributed at fitting interface [8].

5.8.3 Partial Discharges and Erosion Resistance The electrical resistance of insulators to PDs is a very important property for high-voltage applications, such as the stator end windings of rotating machines or wires of randomly wound motors or High-voltage, direct current (HVDC) cross-linked polyethylene (XLPE) cables. PD can create electrical erosion and can gradually erode the insulating materials and lead to breakdown. International Electrotechnical Commission (IEC) electrode and rod-to-plane electrode systems are two main references for evaluation of PD resistance of polymer or polymer-based composites by using several configurations of electrode systems [8]. Nanoparticles as compared to microparticles in the matrix, better fill the spaces and create more torsion and obstacles within the electron passage. The nanofillers can create a better dispersion and distribution

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as compared to microfillers, thereby inhibiting the crossing path between particles, thereby creating additional barriers to PD [8,21].

5.8.4

Space Charge Accumulation

Space charge occurs in a dielectric material when the rate of charge accumulation is different from the rate of charge removal and arises due to a barrier in the path of movement of internal charges or trapping the internal charge such as electrons, holes, and ions. This phenomenon is generally undesirable since it increases the internal field locally within the insulator, thereby leading to a faster and premature failure of the material. Thus, the homocharge materials that have the same polarity and decreases the electric field in the electrode vicinity. Therefore, the localized electric field in the insulator volume is undesirable, because, this create PD in the insulator, thereby accelerating the material degradation and reducing the lifetime. Therefore, a reduction of space charge accumulation is an important goal [8]. In semicrystalline polyethylene, the interfaces between the crystalline and amorphous phases can be considered as charge trapping sites, which are likely to influence the charge accumulation. By the addition of nanofillers, the interface surfaces increase and the interactions between the polymer and nanofiller surfaces increase. These interfaces will introduce or modify the distribution of the trapping sites within the insulator [8]. Determination of the space charge is usually performed by different methods, such as piezoelectric induced pressure wave propagation (PIPWP) method, laser induced pressure propagation (LIPP) method, thermal step method (TSM), and pulsed electro-acoustic (PEA) method. There are some different experimental researches regarding the space charge accumulation in nanocomposites compared to that of microcomposites [8].

5.8.5

Electrical Breakdown

One of the most important properties of high-voltage insulators is electrical breakdown. The incorporation of inorganic fillers into the base polymer can significantly modify the problem of electrical breakdown of the composite material. However, this modification depends on some of specifications of fillers such as the filler loading value, shape, size and surface modifications with different agents, surface chemistry, dispersion, and electrical characteristics of the fillers, materials homogeneity and purity affected by the dispersion of fillers into the base polymer, and the electrical properties of the fillers [8].

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The change of the electric field depends to the difference in dielectric constant (AC) or electrical conductivity (DC) between the inorganic fillers and polymer matrix. As this difference becomes greater, the electrical field distortion increases and the field increase is greater. Therefore, to obtain high-breakdown-strength composites, it must choose the fillers with similar electrical characteristics to the polymer matrix. Since the distortion and enhancement of the electric field is related to the differences in relative permittivity and electrical conductivity between inorganic fillers and organic polymers. Therefore, it can be predicted that the dielectric strength of the polymers can be worsened by simultaneous using high-permittivity fillers (BaTiO3, SiC, ZnO, and AlN) and high electrical conductivity fillers (carbon black, CF and nanotubes, graphite, metals), unless these fillers to be used in applications demanding high thermal conductivity composites [8,70]. Consequently, the similar fillers containing low permittivity and high electrical resistivity are suitable to be incorporated in polymer composite insulators demanding high thermal conductivity and high breakdown strength. Combination of nano and microfillers can also increase the breakdown strength. It was reported that combining BN nanofiller (70 nm, 10 wt.%) with microfiller (500 nm, 1.5 μm, 5 μm, 10 wt.%), leads to an increase in the DC breakdown strength of an epoxy composite. Nanofillers have a considerable improvement in dielectric properties of dielectrics and surface modifications of nanofillers strongly influence on this improvement even when the functionalization of nanoparticles was not performed [8].

Acknowledgment Hereby, Mr. Ilona Ples¸a et al. [8] is appreciated due to the valuable content used in this chapter.

References [1] Mu Liang, KLW, Improving the long-term performance of composite insulators use nanocomposite: A review, in 1st International Conference on Energy and Power, ICEP2016, Energy Procedia 2017 RMIT University, Melbourne of Australia. p. 168
173. [2] Lau KY, Piah MAM. Polymer nanocomposites in high voltage electrical insulation perspective: a review. Malaysian Polym J 2011;6(1):58
69. [3] Lewis TJ. Nanometric dielectrics. IEEE T Dielect El In 1994;1(5).

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[4] Nelson, J.K., Overview of nanodielectrics: insulating materials of the future in IEEE electrical insulation symposium 2006: Toronto. [5] Paramane AS, Kumar KS. A review on nanocomposite based electrical insulations. Trans Electr Electron Mater 2016;17(5):239
51. [6] Nelson JK. Dielectric polymer nanocomposites. New York Dordrecht Heidelberg London: Springer; 2010. [7] Kickelbick G. Introduction to hybrid materials and p. 1
48 Hybrid materials, synthesis, characterization, and applications. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2007. [8] Ple¸sa I, et al. Review properties of polymer composites used in high-voltage applications. polymers 2016;8:173. [9] Taherian R. Experimental and analytical model for the electrical conductivity of polymer-based nanocomposites. Compos Sci Technol 2016;123:17
31. [10] Taherian R, Hadianfard MJ, Nozad AG. A new equation for predicting electrical conductivity of carbon-filled polymer composites. J. Appl. Polym. Sci. 2013;128(3):1497
509. [11] Ali M, Amin M. Polymer nanocomposites for high voltage outdoor insulation applications. Rev Adv Mater Sci 2015;40:276
94. [12] Nan C-W, et al. Physical properties of composites near percolation. Annu Rev Mater Res 2010. [13] Alig I, et al. Establishment, morphology and properties of carbon nanotube networks in polymer melts. Polymer 2012;53:4
28. [14] Alig I, et al. Dynamic percolation of carbon nanotube agglomerates in a polymer matrix: comparison of different model approaches. Phys Status Solidi B Basic Solid State Phys 2008;245(10):2264
7. [15] Schueler R, et al. Agglomeration and electrical percolation behavior of carbon black dispersed in epoxy resin. J Appl Polym Sci 1997;63 (13):1741
6. [16] Sandler JKW, et al. Ultra-low electrical percolation threshold in carbon nanotube- epoxy composites. Polymer 2003;44(19):5893
9. [17] Tiusanen J, Vlasveld D, Vuorinen J. Review on the effects of injection moulding parameters on the electrical resistivity of carbon nanotube filled polymer parts. Compos Sci Technol 2012;72:1741
52. [18] Gao JF, et al. CNTs/UHMWPE composites with a two-dimensional conductive network. Mater Lett 2008;62(20):3530
2. [19] Grady BP. Recent developments concerning the dispersion of carbon nanotubes in polymers. Macromol Rapid Commun 2010;31(3):247
57. [20] Nelson JKFJ. Internal charge behaviour of nanocomposites. Nanotechnology 2004;15(5):586
95. [21] Tanaka T. Polymer nanocomposites as dielectrics and electrical insulation-perspectives for processing technologies, material characterization and future applications. IEEE Trans Dielect El In 2004;11(5) October.

128

ELECTRICAL CONDUCTIVITY

IN

POLYMER-BASED COMPOSITES

[22] Wan Akmal Izzati YZA, Adzis Z, Shafanizam M. Review article partial discharge characteristics of polymer nanocomposite materials in electrical insulation: A review of sample preparation techniques, analysis methods, potential applications, and future trends. E SciWorld J Volume 2014; Article ID735070, 14 page. [23] Jing X, Zhao W, Lan L. The effect of particle size on electric conducting percolation threshold in polymer/conducting particle composites. J Mater Sci Lett 2000;19:377
9. [24] Aneli JN, Mukbaniani OV, Zaikov GE. Physical principles of the conductivity of polymer composites (review). RFP Rubber Fibres Plast Int 2011;6(1):39
49. [25] Pramanik PK, Khastgir D, Saha TN. Conductive nitrile rubber composite containing carbon fillers: studies on mechanical properties and electrical conductivity. Compos Part A Appl Sci Manuf 1991;23:183
91. [26] Al-Saleh MH, Sundararaj U. A review of vapor grown carbon nanofiber/ polymer conductive composites. Carbon 2009;47(1):2
22. [27] Taipalus R, et al. The electrical conductivity of carbon-fibre-reinforced polypropylene/polyaniline complex-blends: experimental characterization and modelling. Compos Sci Technol 2001;61(6):801
14. [28] Tsotra P, Friedrich K. Electrical and mechanical properties of functionally graded epoxy-resin/carbon fibre composites. Compos Part A 2003;34:75
82. [29] Sohi NJS, Bhadra S, Khastgir D. The effect of different carbon fillers on the electrical conductivity of ethylene vinyl acetate copolymer-based composites and the applicability of different conductivity models. Carbon 2011;49(4):1349
61. [30] Buys YF, et al. Utilization of polymer degradation to modify electrical properties of poly(l-lactide)/poly(methyl methacrylate)/carbon filler composites. Compos Sci Technol 2010;70(1):200
5. [31] Rosca ID, Hoa SV. Highly conductive multiwall carbon nanotube and epoxy composites produced by three-roll- milling. Carbon 2009;47 (8):1958
68. [32] Carter RLB. Development and modeling of electrically conductive reins for fuel cell bipolar plate applications, in Chemical Engineering, in Chemical Engineering. Michigan: Michigan; 2008. [33] Clingerman ML. Development and modeling of electrically conductive composite materials. Chemical engineering. Michigan: Michigan technological university; 1998. [34] Dweiri R, Sahari J. Electrical properties of carbon-based polypropylene composites for bipolar plates in polymer electrolyte membrane fuel cell (PEMFC). J Power Sources 2007;171(2):424
32. [35] Keith JM, King JA, Barton RL. Electrical conductivity modeling of carbon-filled liquid crystal polymer composites. J Appl Polym Sci 2006;102:3293
300.

5: NANOCOMPOSITES

IN

DIELECTRICS

129

[36] Ezquerra TA, et al. Alternating-current electrical properties of graphite, carbon-black and carbon-fiber polymeric composites. Compos Sci Technol 2001;61(6):903
9. [37] Kang SJ, et al. Solvent-assisted graphite loading for highly conductive phenolic resin bipolar plates for proton exchange membrane fuel cells. J Power Sources 2010;195(12):3794
801. [38] Mamunya EP, Davidenko VV, Lebedev EV. Effect of polymer-filler interface interactions on percolation conductivity of thermoplastics filled with carbon black. Compos Interfaces 1997;4:169
76. [39] Fothergill J, Nelson JK, Fu M. Dielectric properties of epoxy nanocomposites containing TiO2, Al2O3 and ZnO fillers. Annual report - conference on electrical insulation and dielectric phenomena, CEIDP, 2004. pp. 406
409. https://doi.org/10.1109/CEIDP.2004.1364273. [40] Sigmaaldrich, Kaolin clay. 2017. [41] Alibaba. TiO2. 2017. [42] Sunscreens, Phototoxicity of titanium dioxide in sunscreens, metals in medicine and the environment, site address: http://faculty.virginia.edu/ metals/cases/stclair2.html. [43] Indiamart. Magnesium oxide powder. https://dir.indiamart.com/impcat/ magnesium-oxide-powder.html. [44] Bai W, Zhang Z, Tian W, He X, Ma Y, Zhao Y, Chai Z. “Toxicity of zinc oxide nanoparticles to zebrafish embryo: A physicochemical study of toxicity mechanism. J Nanopart Res 2010;12(5):1645
54. [45] Reade. Zinc oxide powder (ZnO). [46] Reade. Mica flake & mica powder. [47] Mica. Optical and SEM images of silica [SiO2] polymorphs (and varieties). http://minerals.caltech.edu/Silica_Polymorphs/. [48] Praveen AS, Sarangan J, Suresh S, Subramanian JS. Erosion wear behaviour of plasma sprayed NiCrSiB/Al2O3 composite coating. Int J Refract Metals Hard Mater 2015;52:209
18. [49] Reade. Alumina powder (Al2O3). http://www.reade.com/products/alumina-powder-al2o3. [50] EC21, Global B2B marketplace, address site: https://www.ec21.com/ product-details/Spherical-Silica-Powder-0.5-10um–5655770.html. [51] Lempicki A. The electrical conductivity of MgO Single Crystals at High Temperatures. Proc Phys Soc Section B 1952;66(4). [52] Wisz G, Virt I, Sagan P, Potera P, Yavorskyi R. Structural, optical and electrical properties of zinc oxide layers produced by pulsed laser deposition method. Nanoscale Res Lett 2017;12(253):2017. [53] Kadhim MJ, Abdullah AK, Al-Ajaj IA, Khalil AS. Dielectric properties of epoxy/Al2O3 nanocomposites. Int J Appl Innovat Eng Manage (IJAIEM) 2014;3(1). [54] Kasaliwal G, Go€ldel A, Po€tschke P. Influence of processing conditions in small-scale melt mixing and compression molding on the resistivity and

130

[55]

[56]

[57] [58] [59] [60] [61]

[62] [63] [64]

[65] [66] [67] [68] [69]

[70]

ELECTRICAL CONDUCTIVITY

IN

POLYMER-BASED COMPOSITES

morphology of polycarbonate - MWNT composites. J Appl Polym Sci 2009;112:3494
509. Daly HBEN, et al. The build-up and measurement of molecular orientation, crystalline morphology, and residual stresses in injection molded parts: a review. Structure 1998;2(2):59
85. Li CS, Hung CF, Shen YK. Computer simulation and analysis of fountain flow in filling process of injection molding. J Polym Res 1994;1 (2):163
73. Wernik JM, Meguid SA. Recent developments in multifunctional nanocomposites using carbon nanotubes. Appl Mech Rev 2010;63:50801
40. Ramalingam G. Sol gel synthesis of nanomaterials. Slideshare, Publisher: Science; 2014. p. 1
44. Cryston JA. “Sol-Gel”, Book chapter. Comprehensive Coordination Chemistry 2003;1:711
30. wikipedia, Sol-gel, wikipedia, Editor. https://en.wikipedia.org/wiki/Sol% E2%80%93gel_process. Dai H, Wong EW, Lieber CM. Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes. Science 1996;272 (5261):523
6. Nardelli MB, et al. Mechanical properties, defects and electronic behavior of carbon nanotubes. Carbon 2000;38(11
12):1703
11. Hsiou YF, et al. Defect effect on electrical transport of multiwalled carbon nanotubes. Jpn J Appl Phys, Part 1 2005;44(6A):4245
7. Ma P-C, et al. Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review. Compos Part A Appl Sci Manuf 2010;41(10):1345
67. Wu L, Zhang J, Watanabe W. Physical and chemical stability of drug nanoparticles. Adv Drug Deliv Rev 2011;63(6):456
69. M Ceramic processing and sintering. In: Rahaman MN 1382 AP Technology & engineering - 875 pages. CRC Press; 2003. Bodner research web. The Lewis definitions of acids and bases. M. Roy, J.K. Nelson, R.K. MacCrone, et al., “Polymer nanocomposite, et al. Frechette MF, Trudeau M, Alamdari HD, Boily S. Introductory remarks on nanodielectrics. In 2001 Annual report conference on electrical insulation and dielectric phenomena (CEIDP ’01), Canada, 2001. Huang X, Jiang P, Tanaka T. A review of dielectric polymer composites with high thermal conductivity. IEEE Electr Insul Mag 2011;27(4).