Tailoring the properties of epoxy/silicone blends for high-voltage capacitor applications

Abstract

In recent decades, high dielectric constant (k) polymer nanocomposites have proved excellent potential in dielectric and energy storage applications. Epoxy/silicon rubber composite materials have shown promising properties in applications such as high-voltage insulation. Three types of nanomaterials (SiO₂, TiO₂, and TiO₂@SiO₂) with distinct intrinsic properties are carefully chosen to build high-k epoxy/silicone polymer nanocomposites in this study. To raise the value of K, this work tailored the polarizing ability or permittivity of dielectric material by inserting different weight ratios of nano-fillers (SiO₂, TiO₂, TiO₂@SiO₂) in the base matrix of dielectric material in epoxy and silicon rubber blends. With a base matrix containing 75% epoxy and 25% liquid silicone rubber concentration, the maximum value of K obtained is K = 158 for 5% TiO₂ and there is an increase in the dielectric strength to 398 kV/mm. The obtained results indicate that, among the three different kinds of epoxy/silicone, TiO₂@SiO₂ has the most potential in enhancing the energy storage capabilities of the proposed nanocomposites, owing to the largest increase in k while maintaining low dielectric loss and leakage current.

Introduction

Dr. Pierre Castan and Dr. Sylvan Greenlee developed epoxy for the first time in the 1930s. Epoxy compositions used to be predominantly solid, although liquid variants were produced subsequently [1]. Various researches on advanced capacitor dielectrics have been underdeveloped for the last half-century. The demand for smaller and lighter capacitors with high dielectric constant, energy density, breakdown strength, and higher temperature capability increases in the high-voltage industry. Dielectrics are insulator materials with some leaky characteristics. The polarization ability of the dielectrics depends upon their internal chemistry. Materials with higher permittivity have charges that can be more easily displaced. Epoxy resin and silicone rubbers are considered for capacitor dielectrics in high-voltage applications [2]. The properties which make its use attractive are biocompatibility, environmentally friendly, flame resistance, and long shelf-life [3]. With the new developments in nanotechnology, it has been anticipated that the combination of nanoparticles like TiO₂ and SiO₂ with traditional resin systems can create nanocomposite materials with enhanced electrical, thermal, and mechanical properties [4]. An epoxy nanocomposite system exhibits high permittivity at low frequencies and high electrical conductivity when dispersed with conductive fillers [5].

While silicone rubber (SiR) is a lightweight material that possesses high chemical resistance and flexibility with dielectric strength higher than 100 kV/mm [6]; today, SiR is used for volume insulation: high-voltage feed-throughs, generator stator coils, etc. Knowledge of the temperature dependence of dielectric properties from these materials is becoming important for future equipment designs [7]. Various studies show that the addition of nano-fillers (TiO₂, SiO₂, SiO₂@TiO₂) to SiR has significantly improved dielectric constant, dielectric strength, and thermal resistivity, hence leading to the formation of a good-quality dielectric material for capacitors. Thus, using epoxy and SiR can result in a material with enhanced resistivity and dielectric strength for usage at high frequencies and high-voltage capacitor applications.

In [8], it is proposed that the dielectric strength of some dielectric materials can be increased by adding nano-fillers TiO₂ coated with SiO₂.

A system may have a dielectric constant that increases with temperature (3.47 at 23 °C and 3.56 at 100 °C) for a 60 Hz application but changes with temperature (3.29 at 23 °C and 2.98 at 100 °C) for a 1 kHz application [9]. In general, the dielectric constant increases with increased temperatures and decreases with an increase in frequencies. In [10], characteristics are considered in Table 1.

Table 1 Various curing agent groups

SIR is a silicone-based polymer. The properties of silicone rubber in [11] are given in Table 2. It is an important electrical insulating material for nuclear power plant cables [12]. The dielectric constant value for silicone rubber ranges from 2.9 to 4.

Table 2 Properties of silicone rubber

Depending upon their properties and curing time, there are different types of silicone rubber. There is a way to modify dielectric elastomers' properties by adding high-permittivity metal oxide fillers. Liquid silicone rubber possesses a relatively low viscosity favorable for loading inorganic fillers (TiO₂, SiO₂, Al₂O₃) [12,13,14]. Epoxy resin often adds fillers to enhance its mechanical, thermal, and chemical properties. The addition of fillers can deteriorate electrical performance. The dielectric properties and space charge behavior of epoxy resin/nanocomposites with nano-fillers SiO₂ and Al₂O₃ ions [13, 14]. The effect of epoxy species on the tensile and electrical insulation properties of epoxy/micro-silica composites using three species of epoxy resins for high-voltage insulators was investigated in [15]. Polymer nanocomposites are used in power capacitors for electrical energy storage [16,17,18]. Details of nano- and for Electrical Engineering Applications [19, 20]. There is the feasibility of using cassava cortex particles as reinforcement in an epoxy matrix for the development of composite materials with insulation properties (resistivity) [21].

The blend's appropriate weight percentage composition enhanced the dielectric strength and tracking resistance in different conditions [24]. Different dielectric behavior was observed depending on filler type, filler concentration, and temperature [25,26,27]. Variations in real (ε′) and imaginary (ε″) parts of dielectric constants and loss tangent of material with frequency and temperature have been studied [28, 29]. The inclusion of silicone epoxy effectively improved the glass transition temperature (T_g), and the thermal insulation also improved the electrical properties like resistance and dielectric constant for using it as a capacitor at high frequencies and in high-voltage strength applications [26].

The prior work on TiO₂@SiO₂ by [7] was updated in [30] to focus on a limited range of 0.3–0.8% TiO₂@SiO₂ nanoparticles by weight in epoxy. To modify the dielectric characteristics of epoxy resin, a range of nanoparticles have been utilized, including ZnO, Al₂O₃, SiO₂, TiO₂, BN, AlN, and other fillers. In [14, 30,31,32,33], the study looked at the dielectric characteristics of epoxy nanocomposite with low inorganic nano-filler concentrations. The permittivity of the interphase between the polymer and nanoparticles in a nanocomposite has a major influence on the electric field distribution and AC breakdown strength of the nanocomposite, according to [30]. However, further study is needed to fully understand the impact of various nano-filler materials, their percentage loading, and dispersion on the dielectric characteristics of polymers.

Problem statement

For dealing with high-voltage capacitors, some materials have a dielectric between plates of a capacitor with very good dielectric strength. The greater the dielectric strength, the greater the ability to hold the charge between the plates of the capacitor, while a good dielectric means a material with high polarization ability but very low conductivity.

In 2016, Chen et.al [7] proposed that the dielectric strength of some dielectric materials can be increased by the addition of nano-fillers TiO₂ coated with SiO₂. It is proposed in this research to investigate the effect of some other nano-particles on different dielectric materials. Our target will be to tailor the properties of some novel materials like epoxy and silicone rubber for enhanced dielectric constant and dielectric strength. The increase in polarization ability leads to an increase in permittivity, and hence, there is the increase in the dielectric constant of the material.

Work aims to provide the following key benefits by undergoing this research:

• 1. Increasing the permittivity of some common dielectrics.

• 2. Enhancing the dielectric constant of epoxy/SiR blends beyond the values already reported in the literature.

• 3. Increasing the breakdown inception voltage.

Energy storage devices are always required while dealing with high voltages. For that purpose, high-voltage capacitors are designed. But there is always a space limitation that limits the energy storage capacity. Therefore, to cope with this problem, various types of research were made to tailor the dielectric medium between the plates of a capacitor. No further change can be made in the internal material chemistry up to a certain limit. New dielectric materials by introducing three different types of nanoparticles the targeted factors are dielectric constant (K) and dielectric strength. It will be to tailor the properties of some novel materials like epoxy and SiR for enhanced dielectric constant and dielectric strength. The increase in polarization ability leads to an increase in permittivity, increasing the material's dielectric constant. The dielectric properties of epoxy and SiR are tailored by adding three different nanoparticles, e.g., SiO₂, TiO₂, and SiO₂@TiO₂. The applications of this work will be high-voltage (HV) capacitor dielectric materials, compact electric power circuits, HV capacitors used in Marx generators, pulse generators, etc. A pulse duration of a hundred microseconds at a megavolt level to achieve high-energy pulsed power conditioning is used. Examples of loads that require currents in the 100 KA range are electrochemical guns, active armor, directed energy weapons, electric launch platforms, and all-electric warships. Chemical and electrical characterization of the formed nanocomposite was carried out using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), leakage current measurement, relative permittivity, and electrical resistivity measuring methods. The characteristics of epoxy nanocomposites and raw epoxy were then compared. The paper is divided into the following sections which are presented as follows: “Experimental methodology”, “Methods of analysis”, “Results and discussions”, “Conclusion” sections.

Experimental methodology

After deeply going through the literature review, a schematic methodology is designed for carrying out experiments. The base materials are chosen based on their dielectric strength, inertness, and a variety of other factors. Then focus comes on selecting nanoparticle fillers to enhance the strength and properties of sample dielectrics. A proper ratio of materials and filler is defined for each sample composition, leading to nanocomposites' unique characteristics. The detailed view of the research methodology is as follows:

Design of experiment

Experiment design will be accomplished by following steps:

Selection of raw materials

After developing a deep understanding of the nature and properties of materials, it is decided that the base matrix of laminates will consist of epoxy and liquid silicone rubber RTV-528 blends. Epoxy with hardener TETA (Tri-ethylene tetra amine) is selected, being inert and having good dielectric strength and mixing ability. Epoxy/SiR blends are prepared in this research by mixing epoxy and silicon rubber weight ratios. Three different nano-filler types are added to increase the permittivity and hence the dielectric constant of sample blends. Base polymer materials include liquid silicone rubber (LSR-528) and epoxy YD 128 + TETA resin. Nano-fillers SiO₂ nanoparticles, TiO₂ nano-particles, and TiO₂@SiO₂ nano-particles are incorporated.

Epoxy YD 128

YD 128 is an unmodified liquid epoxy resin produced from bisphenol-A and epichlorohydrin and has a medium viscosity. A wide variety of curing agents are available to cure this liquid epoxy resin at ambient conditions. When cured with a suitable hardener, epoxy YD 128 gives excellent chemical, mechanical, and electrical properties. In [30], properties are given in Table 3.

Table 3 Properties of YD128

The relationship between viscosity and temperature for YD128 is given in Fig. 1 from [30].

Fig. 1

Viscosity and temperature relation

Silicone rubber RTV-528

Mold-making SiR, generally named two-part room temperature cured, has exceptional fluidity and good operability. It consists of two parts, flowable liquid silicon, and a curing agent. It gets cured at room temperature 25 °C within 5–7 h in the demolding process. RTV-528 bears good tensile strength, tear strength, and low shrinkage, which is suitable for pouring methods of operation. In [35], the detailed properties are given in Table 4.

Table 4 Properties of RTV-528

The amount of curing agent added decides the time required for silicone rubber to get cured. The approximate values of curing time are given in Table 5.

Table 5 Curing time of different curing agents

Nano-fillers are a class of materials in which at least one component has a nano-sized dimension. By varying the shape, size, and concentration of nanofillers, the macroscopic properties of nanocomposite can be controlled easily [36]. Different nano-fillers will be added to the epoxy/silicone rubber base matrix to improve the permittivity, dielectric strength, and dielectric constant. SiO₂, TiO₂, and TiO₂@SiO₂ are added in varying 1–5% ratios to prepare 45 different composition samples.

TiO₂ is widely used because of its easy availability, nontoxicity, chemical stability, dielectric and optical-electronic properties, low cost, and high photocatalytic properties [37]. Titanium dioxide in ore form possesses a high dielectric constant. The dielectric constant of blend materials depends on the dielectric properties of their components, that is, base polymer matrix and inorganic filler. So, the nanoparticles of TiO₂ with elevated dielectric strength values and low dispersity are strongly recommended for application in high-voltage dielectric materials.

The presence of SiO₂ nanoparticles reduces the conjugation length and thus lowers the conductivity compared to the base matrix alone. Due to the excellent electrical and dielectric properties of SiO₂, it is extensively used in HV capacitors and field-effect transistors. The addition of SiO₂ nanoparticles into epoxy resin enhances the electrical, thermochemical, and optical properties. The addition of SiO₂ nanoparticles into epoxy resin enhances the electrical, thermochemical, and optical properties which are SiO₂ powder and nanoparticle [38,39,40].

Titanium dioxide coated with silicon dioxide

SiO₂-coated TiO₂ nanoparticles are prepared by a sol–gel process. A silica layer is deposited chemically over titanium dioxide nanoparticles in this process. TiO₂ alone possesses high permittivity values, but due to the silica layer above it, the value of permittivity decreases comparatively. This decrease in permittivity is due to the presence of a hydroxyl group in the deposited silica layer. A view of TiO₂@SiO₂ nanoparticles taken from a transmission electron microscope can be seen in the image below. However, this surface modification increases resistivity and polarization ability value.

After selecting raw materials, the standard mixing ratios of the base matrix are calculated based on experiments. Different formulations, F1, F2, and F3, are decided to prepare dielectric discs. It is the weight ratios of two base materials that can be varied by 25% epoxy/75% RTV 528, 50% epoxy/50% RTV 528, and 75% epoxy/25% RTV 528 to form these different kinds of materials.

Filler ratios

After deciding the base matrix ratios, the filler concentration is decided based on previous studies and experiments. Three different filler nanoparticles SiO₂, TiO_2, and SiO₂@TiO₂ are added in 1–5% in base matrix ratio decided above. Thus, 15 unique samples are prepared for each type of filler. For better testing and results, three samples per type are prepared.

Preparing blends

Epoxy/silicone rubber blends are prepared by hand lay-up method. The hand lay-up method is the oldest technique for preparing woven composites. There are following steps involved in preparing the blends. The block diagram of preparation is shown in Fig. 2. At first, a weighted amount of epoxy resin is taken in a beaker. Then, a measured amount of ethyl acetate is added to it with hand blending, followed by 10 mins of magnetic mixing to form a smoothie. Then, a weighted amount of RTV-528 is also taken in a separate beaker to prepare its smoothie with a small amount of ethyl acetate. Ethyl acetate is added to these base materials to decrease their viscosity so that they can blend up with each other to form a uniform mixture. Lowering the viscosity, better and easier the mixing of materials gives a uniform mixture. The liquids (epoxy resin + LSR) are recently mixed and set for ultra-sonication for 20 mins. There is the blend of two materials with uniform mixing at the particle level.

Fig. 2

Block diagram for preparation of blends

The next step is the dispersion of nanoparticles for the reinforcement of blends. A very carefully weighted amount of nanoparticles is added to a small amount of ethyl acetate to prepare a dispersion. These dispersed nanoparticles are then added to the base matrix mixture and set for ultrasonication for 20 mins. The prepared mixture is ready to be cast in molds. Figure 3 describes the formulation blends steps.

Fig. 3

Formulation of blends

Methods of analysis

Characterization of specimens

The prepared dielectric disks are passed through various analysis methods for their characterization, capacitance measurement, dielectric strength measurement, leakage current measurement, breakdown current measurements, etc. Depending upon these characteristics, it will be determined how good our prepared dielectric material is.

Measurement of capacitance

The prepared dielectric disks are 5 cm in diameter. To form a capacitor, two 6-cm-diameter metal discs are placed on both sides of the dielectric disk to form a capacitor as shown in Fig. 4. The mode is set to capacitance measurement, and the scale is adjusted at nF. The measurement process is shown in Fig. 5.

Fig. 4

Dielectric between plates of a capacitor

Fig. 5

Capacitance measurement

Resistivity is the intrinsic property of a dielectric material. It is the measure of opposition offered by a material to the flow of electric charges in a particular dimension [24]. Resistivity can be measured easily with digital multi-meter (DMM). The unit is set at giga-ohm, and the resistivity of all sample dielectric disks is measured.

Dielectric constant

The dielectric constant is a factor that determines the capacitance of a dielectric material. It is a ratio of two capacitances: capacitance of material and capacitance of free space, given by:

$$\begin{array}{*{20}c} {K = \frac{{C_{{{\text{material}}}} }}{{C_{0} }}} \\ \end{array}$$

(1)

$$C = KC_{0}$$

(2)

$$\begin{array}{*{20}c} {C = \frac{{KA\varepsilon _{0} }}{d}} \\ \end{array}$$

(3)

$$K = \frac{{Cd}}{{A\epsilon _{0} }}$$

(4)

Here the value of ${\epsilon }_{0}=8.854\times {10}^{-12}$ CV⁻¹ m⁻¹. The dielectric constant values are calculated by putting the values A = 19.63 cm² and d = 2 mm in (4). The energy stored in a capacitor depends on the dielectric constant—greater the stored energy for greater values of dielectric constant K.

Our prepared dielectric disks are tested at a high-voltage laboratory to determine their maximum dielectric strength. The apparatus is installed in the laboratory setup as shown in Fig. 6. A 220 V source supply is provided to the control unit, where two sections: low voltage measuring input and regulated voltage. Then, a 100 KVA transformer is connected in series. A current limiting resistor is connected in series with the transformer, and then, a diode rectifier is connected. The sample to be tested is clamped after the rectifier, another end is grounded, and a high-voltage reading comes to the display at the HV measuring unit.

Fig. 6

Dielectric strength measurement

Thermal conductivity

Thermal conductivity measures how much heat energy the sample conducts in case of capacitor heat-up. The greater the thermal conductivity, the higher the heat dissipation rate of the material. Good thermal conductivity prevents the equipment from being damaged by dissipating the generated heat in the surroundings. The sample is placed above heated plates, as shown in Fig. 7. The temperature is raised to 200 °C, and conduction is measured at the other end. The assembly for heat measurement is shown below:

Fig. 7

Testing thermal conductivity

The dielectric disks are exposed to fire flame for 60 s approx. A slight black carbon layer was deposited over the spot where the flame touched the sample.

Results and discussion

Different behavior was observed for three different types of reinforced nanoparticles upon measuring the capacitance. The maximum capacitance values were found for TiO₂ nanocomposites. At a 5% wt ratio of TiO₂, the maximum value was achieved, which is 1372.4 PF, while minimum values were recorded for SiO₂ nanoparticles.

Electrical parameters of materials are compared in Fig. 8. A comparison of capacitance values is made based on the graphs shown in Fig. 8a. It is observed that the capacitance value usually increases with the increase in nano-filler ratios. This trend was not found in the case of SiO₂, where capacitance values decreased after a 3% wt ratio. At the same time, the highest capacitance values can be seen for 5% TiO₂ and 5%TiO₂@SiO₂.

Fig. 8

a Capacitance comparison of different materials, b dielectric constant comparison of different materials, c dielectric strength comparison of different materials, d leakage current comparison of different materials, e resistivity comparison of different materials

The dielectric constant values are calculated using Eq. (4) described above. Dielectric constant K is a direct measure to determine the quality of a dielectric material. Higher K values represent the greater energy storage capacity of the dielectric medium. The maximum value achieved by our research is K = 158 for a 5% wt the ratio of TiO₂. It was also observed that TiO₂@SiO₂ samples also possess good values of K as compared to SiO₂ dielectric samples. A comparison made for K values is given in Fig. 8b. The maximum value for TiO₂@SiO₂ samples is K = 153.4. K values for three types of particles are represented below. It is found that SiO₂ has more insulative properties and less polarization ability as compared to TiO₂ nanoparticles. Because of less polarization ability, SiO₂ has less permittivity, which leads to less dielectric constant values. At the same time, TiO₂ nanoparticles possess more polarization ability and thus higher K values.

The recorded values of dielectric strength, dielectric constant, leakage currents, and resistivity of three types of particles are given in Table 6.

Table 6 Electrical properties of different materials at different concentrations

When designing the dielectric for a high-voltage capacitor, you must take into account the higher voltage by using a material with high dielectric constant and dielectric strength values. high-voltage capacitor dielectric, deal with high voltage, so such as a material with high values of both dielectric constant and dielectric strength. It is observed that TiO₂ particle samples had more dielectric constant than the two other types of samples. But at the same time, the dielectric strength values of TiO₂ samples were less than TiO₂@SiO₂. Therefore, the best option available was TiO₂@SiO₂ nano-filler samples, with a high K value of 153.4 and a high dielectric strength of 398 kV/mm. So, it was found that using TiO₂@SiO₂ nanocomposites gave a high value of dielectric constant K more than SiO₂ and dielectric strength more than TiO₂. Therefore, using 5 wt% of TiO₂@SiO₂ gave the best possible choice for HV capacitor dielectrics. These samples also had the least leakage current and comparatively less resistivity.

Conclusions

The addition of a minimal amount of TiO₂@SiO₂ nanoparticles to epoxy resin improved the dielectric characteristics of the nanocomposite compared with pure epoxy resin. There was an increase in dielectric constant, resistivity, dielectric strength, and leakage current densities. It was also discovered that a low ratio of nanoparticles produces an important influence on the dielectric characteristics of the base material due to the high surface energy of nanoparticles. The highest capacitance value was observed for TiO₂ nanoparticle sample dielectrics. While looking at the dielectric strength of TiO₂, that was the least as compared to the other two types of samples. Leakage current and resistivity values are also low for TiO₂.

Furthermore, due to the high surface-to-volume ratio and high surface energy of nanoparticles, filler concentration plays a crucial impact in defining the dielectric characteristics of nanocomposites. As a result, it is proposed that increasing the filler concentration in fractional phases is explored to discover the optimal loading concentration for the best dielectric characteristics.