Synthesis and Diagnostics of Nanostructured Micaless Microcomposite as a Prospective Insulation Material for Rotating Machines

: This paper deals with the topic of composite insulation materials for rotating machines and it is primarily pointed to the synthesis of new three-component insulation system. In connection with this research, the basic components of the insulation system are selected and described by different diagnostic methods. The proposed insulation material is composed of epoxy resin based on bisphenol-A diglycidyl ether, magnesium oxide nanoﬁller (1 wt %) with its own surface treatment technology using epoxysilane coupling agent ( γ -glycidoxypropyltrimethoxysilane) and polyethylene naphthalate as a reinforcing component. Following the deﬁned topic of the paper, the proposed three-component insulation system is confronted with commonly used insulating systems (PET reinforced and Glass reinforced mica composites) in order to verify the basic dielectric properties (dielectric strength, volume resistivity, dissipation factor) and other parameters determined from phenomenological voltage and current signals, respectively.


Introduction
Dielectric materials of different forms and states are inseparable parts of high voltage applications. From the point of reliability, it is the weakest part of the reliability chain of the whole device [1]. For this reason, it is necessary to take an effort to the improvement of material parameters (electrical, mechanical or physical-chemical). It leads to the reduction of the probability of failure of the entire device [2]. Current trends in the field of design and diagnostics allow the usage and study of the materials containing components up to nanometric dimensions [3]. One way to achieve the desired improvement is to use composite materials [4] where the synergy effect of individual components is utilized. In the field of insulating systems for rotating machines, there are, in particular, three component composite materials, which are composed of a matrix, filler and reinforcement [5].
This area of composite insulation systems has undergone considerable development over the past 70 years. In the case of insulation systems for rotating machines, natural fibers from materials such as silk, cellulose, and linen were initially used. Natural varnishes from trees, plants and insects, or petroleum derivatives [6] were used as an impregnants.
Progress in the technology of composite insulating materials of rotating machines is dated to the time of use of mica leaves in unaltered form with cellulose reinforcement binded together with shellac or asphalt matrix [7]. This type of insulation is also known as a "soft" insulating system.
The undisputed advantage of soft systems was their elasticity and the ability to fill the machine slot, completely without the spaces [8]. One of the main reasons for their replacements were limits in terms of the operating thermal class and voltage level or partial discharges developed in delamination due to the shrink.
Another milestone in this area was the incorporation of mica paper as a part of the insulation system in the middle of the 20th century [9]. At that time, also a development, research and application grew in the field of impregnants. Previously used bituminous varnishes being slowly replaced by synthetic resins. The most common representatives for electrotechnical applications are polyester [10], epoxy [11] and in special cases silicone [12] resins. Also natural fibers have been replaced over the time. Glass fibers, polyethylene terephthalate or polyethylene naphthalate films [13] are used today as a reinforcement. These so-called "hard" insulation systems can be operated at higher temperatures and voltage levels. Their disadvantage may be the fact that they do not sufficiently fill the slot of the machine, which in some cases may lead to the formation of slot partial discharges. The illustration of the stator winding with composite insulation is shown in Figure 1. Another significant contribution from the historical perspective on the development of composite dielectrics and their electrical properties can be attributed to Johnston and Markovitz [14]. They used a nanometric fillers in corona resistant insulation at the beginning of the 1990s. As a next, Lewis defines the uniqueness of nanodielectrics in his article in 1994 [15]. Then, the nanodielectric systems become an object of interest of scientists and professionals from all over the world, who are discussing other options for modifications and potential applications [16]. The most frequent studied fillers ( Figure 2) are simple oxides (SiO 2 [17], Al 2 O 3 [18], ZnO [19], TiO 2 [20], MgO [21]), layered [22] or tubular [23] silicates, eventually also nonwoven nanoscale structures [24][25][26]. With the advent of last mentioned technologies, it is possible to characterize the composites as macro-, micro-and nano-. Globally, the macrostructural insulation system-macrocomposite-can be defined as a system containing in its structure components with dimensions larger than 100 microns in the transverse direction [27]. Analogously, the term micro-or nanocomposite, can be also defined. Because of the dimensions of the used components, these terms refer to commonly used three-component insulating systems for electrical rotating machines. The homogeneity level of the internal structure may be defined according to Figure 3. By replacing one macro-or microscopic interface (e.g., by dispersed nanoparticles), the formation of the so-called nanostructured microcomposite [16] can be achieved by the combination of the components of both characteristic dimensions (nano-, micro-). As has been already mentioned, the main aim of this paper is to introduce a new three-component micaless insulation system with the potential for usability in electrotechnical applications. This material has promising dielectric properties (dielectric strength 58 kV/mm, volume resistivity in the order 10 14 Ω·m and dissipation factor in order 10 −3 ) in comparison with commonly used insulation systems for rotating machines. The raw materials, production process and subsequent diagnostics of synthesized nanostructured micaless microcomposite are described in the following text.

Matrix
The matrix, or binder, is any material or substance that connects or saturates the individual phases of the composite, resulting in a coherent whole. The basic requirement for a suitable matrix is the level of its adhesion with the filler and the reinforcement, as well as the wettability and chemical inertness of the interphase regions of the individual components [28].
Industrial epoxy resin Epoxylite 3750 LV (produced by Elantas) has been selected as a suitable matrix. It is based on bisphenol-A epichlorohydrin, which are essential components for the polymerization of bisphenol-A diglycidyl ether ( Figure 4). Epoxylite 3750 LV is used in industry due to its properties [29], such as low processing viscosity (in our case ensures better filler dispersion [28]) and high bond strength. It is used by manufacturers for the impregnation of windings of electrical machines by vacuum-pressure technology (VPI). Epoxylite 3750 LV is curable at elevated temperatures (140 • C, 4-6 h or 160 • C, 3-6 h) without additional hardener at it is free of additional solvents, which could negatively influence the interaction between matrix and filler, or reinforcement, respectively.

Filler and Coupling Agent
The filler is generaly an element added to the base material, most often for improvement of some its properties. More than 50 million tons of fillers are used annually in different application areas according to marketing study [30].
Magnesium oxide (MgO) in the form of nanoparticles with an average size of 20 nm (NanoAmor) [31] has been chosen as filler, due to its chemical and physical stability, refractoriness and also due to its excellent electrical insulating properties [32]. Magnesium oxide occurs in nature as mineral periclase [33] in contact metamorphic rocks.
Taking into account the preliminary measurements of the main electrical properties, which are deeply described in [32], the optimum weight ratio was set to 1%. Especially due to the values of dielectric strength and volume resistivity. The box plots with marked average values of investigated parameters are shown in Figure 5.  For the improvement of the mixability between filler and matrix, the silane coupling agent γ-glycidoxypropyltrimethoxysilane (Sigma-Aldrich) has been selected, hereinafter referred to as GLYMO. It is an epoxisilane coupling agent, specifically an organofunctional trialkoxisilane utilizing high reactivity between epoxy rings and amino groups [34]. The optimal amount of coupling agent depending on its and filler properties and it has been calculated to 18.12% of filler weight. The effect of addition of surface coupling agent is presented on selected dielectric parameters ( Figure 6).
As has been already mentioned, more detailed information about the MgO, its dielectric properties and possibilities of surface modification by γ-glycidoxypropyltrimethoxysilane could be found in our previous studies [32,35].

Reinforcement
The reinforcement, or carrier component, is understood to mean the part of the composite material having primarily the task of increasing the mechanical strength and stiffness of the material [36]. In electrical applications, the reinforcement may also behave partially as a dielectric barrier. This is especially valid for systems containing a film reinforcements.
Currently, the market has a relatively wide range of reinforcements. Especially woven, film, foil or fiber reinforcements are used for electrotechnical applications.
The most interesting choice for the synthesized material was the polyethylene naphthalate (PEN) film Teonex Q51 (Teijin DuPont) of a nominal thickness of 25 µm. This material ensures the high electrical strength of the resulting composite. The electrical strength of the PEN Teonex Q51 film is 300 kV/mm (valid for 25 µm) [37].
In general, polyethylene naphthalate is a linear polyester with excellent electrical and mechanical properties. The basic materials for its production are naphthalene dicarboxilic acid and ethylene glycol [38]. Compared to PET, which is commonly used as a reinforcement, PEN exhibits significantly better properties. These are higher chemical and hydrolytic resistance, higher Young's modulus, lower oligomer extraction, and higher glass transition temperature [39][40][41][42]. These different material properties are due to differences in chemical structure. While PET contains terephthalic acid, which has only one benzene nucleus, PEN contains naphthalene dicarboxylic acid, which has two linked benzene nuclei (naphthalene) [39]. Due to its considerably better properties, it becomes a suitable substitute for commonly used polyethylene terephthalate, not only in the electrical industry [43].

Synthesis of Nanostructured Micaless Microcomposite
In the previous parts, individual components, dielectric properties and their application advantages were described. In this section, attention will be paid to the description of the production process of synthesized composite material. It can be divided into several phases, which are shown in block diagram in Figure 7.
In general, particles or films often contain a significant amount of water molecules which are absorbed or weakly bonded in the volume. From this reason, it is necessary to start with the dehydration. Magnesium oxide powder placed in Petri dishes was dried in a hot air dryer together with sheets of polyethylene naphthalate film (80 • C, 24 h) to eliminate the effect of moisture. After that, the particles were added to a solution of 96% ethanol 4% H 2 O (10 mL) and ultrasonic stirred (20 kHz, 30 min). The surface functionalization process follows with addition of GLYMO modifier (18.12%) to the mixture with re-application of ultrasonic stirring (20 kHz, 2 h). Meanwhile, the epoxy resin was heated to a temperature of 75 • C, which resulted in a lower viscosity for subsequent processing. The surface-treated (ST) nanoparticles were then added to the already heated epoxy resin and subsequently magnetically stirred (600 rpm, 3 h, 75 • C). Last but one step was combined ultrasonic and magnetical stirring (300 rpm, 20 kHz, 30 min, 70 • C). Finally, vacuum venting was performed with reduced magnetic stirring (300 rpm, 3 h, 90 • C). The elevated temperature of 90 • C provided evaporation of the ethanol from the epoxy mixture. The resultant mixture was then applied to preheated Teflon molds with a silicone spacer and a PEN reinforcing component. Preheating of Teflon molds eliminates the effect of their surface moisture and ensures better adhesion of the silicone spacer. Considerable attention has been also paid during production to ensuring the homogeneity of the internal structure of the sample by removing the resulting air bubbles. The prepared samples have been cured in a hot air dryer (140 • C, 6 h).  Figure 7. Block diagram of composite preparation.

X-ray Diffraction Analyses of Synthesized and Commonly Used Composites
The X-ray diffraction measurement has been used for material identification [32,44]. These measurements have been performed on a Panalytical X'Pert Pro (Malvern Panalytical) automated powder X-ray diffractometer using an X-ray lamp (IKa1 = 0.154 nm, 40 kV, 30 mA) and a semiconductor ultra-rapid PIXcel detector in the geometric Bragg-Bretan arrangement.
From the X-ray diffraction analysis is visible the character of the amorphous material (matrix). This symbolizes very wide diffraction (6-8 degrees in the diffraction angles 2-theta), which are shown in Figure 8.
These diffractions, resp. their positions on the x-axis, corresponding with the results of presented studies [45,46]. On this diffractogram, only the diffraction pattern of MgO (200), (220) and (222) are noticed. Other lines are weak, or we do not notice them at all. Analysis of the profile of diffraction lines (200) showed that the size of the coherent dispersion region of X-ray crystallization (crystallite) is in all cases about 23-25 nm and the micro-deformation is relatively low (0.0022-0.0025). This fact corresponds to the knowledge of the average size of nanofillers from manufacturer [31].
From the results of synthesized nanostructured microcomposite is obvious difference in the area of the first wide diffraction of the epoxy matrix and also in the decrease in the diffraction lines of the magnesium oxide in comparison with pure epoxy resin and resin with dispersed MgO, respectively. When comparing the position of this diffraction change with study [47], the behavior corresponding to the polyethylene naphthalate (about 26 degrees in the range of 2-theta diffraction angles). Decreases in the intensity of individual diffractions are due to the presence of PEN foil and it indicates that the volume ratios of composite have been changed. The attention has been also paid to the analysis of commonly used insulation materials. There are visible peaks of used filler, which corresponds to Muscovite 6-263(2M1) whose has been investigated e.g., in [48]. Unfortunately, due to the majority share of mica paper in composite and its intensity, it is not possible to define other elements using this technique. However, slight differences between glass reinforced composite and PET reinforced composite are visible in Figure 9.

Methods
In the following sections, the methods and measurement setups are described. The collection of 3-5 samples were used for each measurement. Flat samples have a square shape and dimensions depending on the requirments of the selected method (100 × 100 mm or 40 × 40 mm).

Broadband Dielectric Spectroscopy
Broadband Dielectric Spectroscopy (BDS) is a modern diagnostic method which allows interconnecting several measurement techniques to obtain a comprehensive view of the material behavior under an electric field with a frequency in a very wide range. For this investigation, the main diagnostic unit of the Alpha-A measuring device (Novocontrol Technologies) has been used. It contains a frequency response analyzer with a sinusoidal signal generator. The presented analysis was performed in the temperature range from −50 • C to 60 • C and in the frequency range from 0.5 Hz to 1 MHz.

Resorption Current Analysis
The aim of this section is to introduce the methods of dielectric resorption analysis. Namely a method of determination of the area over the curve and the method of the tangent curve determination. These procedures are presented in publication [53] in connection with the analysis of dielectric absorption. However, in this case, it will be used for analyses of the behavior after DC voltage disconnection.
The electrometer Keithley 6517A (Keithley Instruments) with 8009 Resistivity Fixture (Keithley Instruments) electrode system were used for this measurement (schematicaly in Figure 10a). The measurement starts with the charging (3600 s) and after that, the dielectric resorption current was measured from 0 to 300 s. For better comparison between individual composite materials, both parameters have been determined at the applied intensity of electric field 1 kV/mm. In the following text, the main ideas of above-mentioned techniques will be described. Area over the curve gives also the information about the ability of the material release accumulated charge that has been accumulated inside the material structure due to the DC voltage application.
Whereas the interval was set within the range of 0 to 300 s, it also allows the inclusion of dielectric resorption in the time immediately after disconnection from DC voltage source. Graphical description of mathematical processing is illustrated in Figure 10b. Because there are discrete values of the resorption current, the area above curve can be defined as an area of few trapezoids.
In the context of measurement, the data can be analyzed by another technique. The time constant of the dielectric, which is given by multiplicity of the resistance and the capacitance dielectric, can be determined by the construction of the tangent curve (Figure 10c).

Partial Discharges Measurement
The experimental measurement was carried out using a galvanic measurement method in accordance with IEC EN 60270 standard [54] with a parallel measuring impedance ( Figure 11). The measured samples have been immersed in an oil bath for the elimination of surface discharges which could overlap the internal discharge activity [55,56]. High voltage source 200 kV (High Volt), coupling capacity 1000 pF ± 10% (Maxwell Technologies), measuring impedance LDM-5 U 50 Ω (LDIC), measuring and evaluation device PD Smart (Doble Lemke), pulse calibrator LDC-5/S3 (LDIC) and electrode with rounded edge limiting the effect of corona discharges have been used for this investigation. For better comparison, the inception voltage was replaced by the inception level (kV/mm). Values of charge Qiec (pC) have been determined at the inception level after 60 s. Following the results of the measurements, the comparison at the level 18.5 kV/mm has been performed.  Figure 11. Partial discharges measurement circuit according to IEC 60270 Std.

Results and Discussion
In this section, the results achieved by previously describes methods are presented and discussed. The synthesized nanostructured micaless microcomposite is compared with two commonly used insulation systems [57,58] for verification of its properties.

Key Dielectric Parameters
The key dielectric parameters have been investigated according to the above-mentioned standards and test conditions (Table 1) at Standard conditions B given by IEC 60212:2010. From the probability plots ( Figure 12) is visible that the measured values show a low level of variability. There are also noted major statistical parameters, like average value (µ), standard deviation (σ) and coefficient of variation (θ).
For the comparison with the study of Harvanek [28] is necessary also included minimum measured values (maximum value for dissipation factor). These values are shown in Table 2. From this comparison, it is clear that the synthesized material (marked A) exceeds in all cases commonly used composite materials contains mica paper (PET reinforced mica composite marked B, Glass reinforced mica composite marked C). It confirms the appropriate selection of individual components of the synthesized nanostructured micaless microcomposite.
There is visible that the minimal dielectric strength of film reinforced composites (A, B) is relatively equal, but higher for synthesized nanostructured micaless microcomposite. It is caused by the compact character of the reinforced component, which fulfills the role of a dielectric barrier next to the reinforcing [59]. The positive effect of a polymer film on dielectric properties is also mentioned by the authors of the studies [60,61].

Figure 12.
Normal probability plots of investigated parameters of nanostructured micaless microcomposite.

Sample min E p (kV/mm) max tan δ (-) min ρ v (Ω·m)
A-Nanostructured micaless microcomposite 53.2 0.00452 4.84 × 10 14 B-PET reinforced mica composite [28] 50 0.015 1 × 10 13 C-Glass reinforced mica composite [28] 35 0.015 1 × 10 13 Other parameters are influenced by the internal structure of the composite, where primarily calcined mica paper contributes to dielectric losses by its strongly inhomogeneous structure, even though it is composed of smaller platelets than non-calcined mica paper [62]. By introducing the MgO into epoxy resin, the different energy levels are achieved. It resulting in the differences in dielectric losses and volume resistivity.

Broadband Dielectric Spectroscopy
It is evident from the results (Figure 13) that the synthesized material (A) shows the stability of the real part of complex permittivity and its value is lower in comparison with commonly used insulation systems (B, C). This fact is very important for potential application in the industry. The lower value of real part of complex permittivity, which represents relative permittivity of insulation could cause lower local overloading if the delamination occurs. This fact can be easily illustrated by using of Maxwell-Wagner model for double layer insulator. In the case of presence e.g., air-filled delamination, the maximal field strength in the cavity is lower if the surrounding material has also lower permittivity.
If the attention is focused on the dielectric processes it is very difficult to precisely define the individual polarizations which are visible in the imaginary part of the complex permittivity, because there is a combination of thermoplastic reinforcement with a thermosetting resin. However, these polarizations are probably caused due to the local motion of small molecular units, side chains and aromatic substitution groups, which contribute slightly to the dielectric losses [63,64] or due to the Brownian motion of the chain segments [65] in the case of higher temperatures. In the case of a loss factor, it is visible that the surfaces of investigated materials overlap to a large extent.

Resorption Current Analysis
Resorption current analysis brings an interesting view on the response of internal structure after DC voltage application. It is evident from the results ( Table 3) that the proposed nanostructured microcomposite has the highest ability to dissipate the accumulated charge in the internal structure after disconnection from the DC voltage source. This difference can be attributed to the substitution of the highly inhomogeneous mica paper interface with the nanometric magnesium oxide filler at optimal weight ratio. Another reason is to increase the density of deep electron traps, in which the captured charge carriers can easily recombine [66,67]. This fact is strongly connected with conductivity of material. It is based on the Poole-Frenkel mechanism, wherein the deep traps enhance the height of energy barriers [68]. From this reason, the introduction of deep traps could be considered more helpful for insulating materials than shallow traps [69]. Table 3. Parameters determined from resorption current.

Partial Discharges Measurement
The partial discharges (PDs) are one of the most occurring phenomena in composite insulation systems, especially in rotating machines. From this reason, the PD inception level and charge Qiec has been investigated. The obtained results are shown in Table 4. The character of the discharge activity at comparative level of 18.5 kV/mm is shown in the form of Phase Resolved Partial Discharges (PRPD) diagrams ( Figure 14). From these diagrams, it is evident that their character corresponds with the internal partial discharges. There are also very well visible the movements of the "rabbit ear" [70] during the changes of the applied voltage, which are indicators for void [71] partial discharges. The difference in discharge activity could be attributed to the strong inhomogeneity of the internal structure or imperfect adhesion between the various parts of commonly used composite materials. Here, small, gas-filled areas near fibres, film or mica paper could occur during the process of producing the insulating system [72][73][74].

Conclusions
In this paper, new nanostructured micaless microcomposite insulation material has been synthesized and diagnosed. The basis of the synthesized material is Epoxilite 3750 LV solvent-free epoxy resin, which is commonly used in industry to impregnate rotating machines. Mainly due to its low viscosity and high bond strength. The better properties of epoxy resin have been achived by the addition of optimum amount of magnesium oxide filler. This was set according to preliminary tests of dielectric parameters. Magnesium oxide is a simple oxide with the highest internal resistivity of commonly used oxides. Polyethylene naphthalate film was used as a reinforcement mostly for the reason to increase dielectric strength of whole composite.
The synthesized nanostructured micaless microcomposite has been also subjected to different diagnostic methods. All of these tests have been performed for two representatives of commonly used three-component systems for comparison. From this investigation is evident, that there is a possibility of eliminating the mica paper, which could lead to better insulation properties.

Abbreviations
The following abbreviations are used in this manuscript: