Dielectric properties and corona resistance of Si-B/epoxy nano-composites
In order to meet the increasing requirements for materials in the electrical and electronic industry, in this paper, Si-B oxides were designed and synthesized via the organic monomers of Si and B, and the Si-B/epoxy nanocomposites were prepared by ultrasonic dispersion and thermal curing. The Si-B oxides were dispersed uniformly in epoxy resin with great compatibility and the average size of the particles was about 6 nm. Results showed that with the increase of doping content, the dielectric constant and dielectric loss of the Si-B/epoxy nanocomposites decreased, but the effect on the conductivity was not obvious. The dielectric strength and corona resistance life of the Si-B/epoxy nanocomposites increased at first and then decreased. When the doping content was 6 wt%, the corona resistance life of the Si-B/epoxy nanocomposites reached up to 17.6 h, which was 8.38 times of that of pure EP. It was proved that the dielectric properties and corona resistance of epoxy could be effectively improved by doping the Si-B oxides.
Epoxy resin (EP) is widely used due to its excellent electrical properties, such as motor, power electronic devices, integrated circuit packaging, etc. [1, 2, 3]. For dielectric materials used in the circumstance of high electric field, they should have good dielectric properties and corona resistance. Many insulating materials have little difference in dielectric strength, but their corona resistance is quite different. For applications under high electric field, corona resistance is particularly important. Therefore, it is of theoretical and practical significance to improve the corona resistance of dielectric materials.
Inorganic/organic nano-composite polymer materials have attracted wide attention due to their excellent properties, which combine the advantages of organic polymers and inorganic materials [4, 5, 6, 7]. It is reasonable to believe that they have potential to be developed into new materials with special properties [8, 9, 10]. In the research of EP composites, there are many studies on their dielectric properties, and the commonly used nanoparticles are SiC , SiO2 [12, 13], ZnO , Al2O3 , TiO2 [16, 17]. The addition of nanoparticles improves the thermal stability, mechanical properties and electrical properties of EP composites for varying degrees. However, there are few reports on corona resistance of EP materials [18, 19, 20].
At present, it is difficult for single polymer or single filler filled polymer composites to meet the requirements of multifunctional properties. The pool compatibility between inorganic nano fillers and organic matrix leads to the agglomeration of traditional nano-fillers [21, 22, 23]. Besides, the surface functionalization of nanoparticles usually make the preparation process more complex. In this paper, nanometer Si-B oxides were synthesized by Si and B organic monomers, and Si-B/EP nanocomposites were prepared by blending method. The morphology of Si-B/EP nanocomposites were characterized. The effect of Si-B oxides content on dielectric properties of materials were analyzed, and the factors affecting corona resistance were discussed emphatically.
2 Materials and methods
The following materials were obtained from the respective companies: phenyl triethoxy silane(PTES), from Hubei Ju Sheng Technology Co., Ltd, China; tributyl borate, from Tianjin Guangfu Fine Chemical Research Institute, China; distilled water, self-made; ethanol absolute, from Tianjin Tianli Chemical Reagent Co., Ltd, China; Epoxy resin used was bisphenol A diglycidyl ether (BPADE), which was prepared through the molecular distillation technology by Suzhou Jufeng Electrical Insulation System Co., Ltd, China, its polymerization degree was 0, and the epoxide equivalent was 170 g/equiv; reactive diluent: glycidyl neodecanoate(product model: E10P), from hexion specialty chemicals, Holand; curing agent: latent curing agent 594(594), from Wuxi Qianguang chemical materials Co., Ltd, China; imidazole, from Tianjin kemi’ou Chemical Reagent Co. Ltd.
2.2 Preparation of Si-B oxides
43.2 g of PTES and 4.6 g of tributyl borate (mole ratio 9:1) were taken in three mouth bottles. In the case of high-speed stirring, 50% ethanol solution was dropwise added to the bottle until the quality of the water reached 10.08 g. Under the condition of reflux, reactants were heated to make PTES and tributyl borate hydrolyze for 4 h at 80 °C. Then the reflux was changed to distillation and the temperature gradually increased to 120 °C. When the temperature reached 120 °C, the residual ethanol and water in the Si-B oxides were removed under vacuum distillation, and then the reaction was complete.
2.3 Preparation of Si-B/EP resin and Si-B/EP composite specimens
Pre-weighed 594 and the obtained Si-B oxides were added into three mouth bottles. The mixture was stirred until it was uniform and transparent. And then, pre-weighed BPADE were added and continued stirring until the mixture was transparent and uniform. Finally, ultrasonic treatment was performed for 30 min after diluent and imidazole were added. Colorless transparent Si-B/EP nanocomposite resin was prepared. Before used, the resin should be filtered by 1000 mesh strainer and then the bubbles in the resin would be removed in the vacuum oven.
Preparation of Si-B/EP nanocomposite disc specimens: the disc mold of stainless steel was coated with mold release agent and heated in a 150 °C oven. A certain amount of Si-B/EP nanocomposite resin was poured into the heated mold. It was found that the EP resin could crosslink by heating as follows: 150 °C for 1 h, 170 °C for 12 h. Finally, the Si-B/EP composite specimens were obtained. All of the as-prepared composite specimens had a carefully controlled thickness of 1 ± 0.1 mm. Preparation of thin film specimens for corona resistance testing was as follows: A coating of the prepared mixture was brushed onto a clean aluminum foil paper which was attached to clean glass panels. The same heating program was executed as the disc specimen. The thickness of the film wad controlled at 25 ± 1 μm. Samples of Si-B/EP nanocomposites with 0 wt%, 4 wt%, 5 wt%, 6 wt% and 7 wt% doping content were prepared.
The morphology of the Si-B oxide nanoparticles was observed using a transmission electron microscopy (TEM, JEM-2100). The relative permittivity of the Si-B/EP nanocomposite was measured using a Novocontrol Dielectric Spectrometer (GmbH Germany), CONCEPT 40 in a frequency range from 100 to 106 Hz. Prior to dielectric measurements, aluminum electrodes were deposited onto both surfaces of the specimens by sputtering. The diameter of aluminum electrodes was about 30 mm. The thickness of all specimens was about 1 mm. Conduction current characteristics were tested from 10 to 80 kV/mm. HT-100 breakdown voltage tester (Guilin Institute of Electrical Appliances) was used for testing breakdown field strength of composite materials. The frequency of the applied voltage was 50 Hz. Dielectric strength was calculated as the electrical voltage divided by the sample thickness at the breakdown point. Since the short-term breakdown properties of solid dielectric shows a very strong dependence on the electrode distance. The thickness of specimens with diameter of 100 mm was controlled around 1 mm. Corona resistant test electrode system was made in accordance with IEC60343  standard. Tests were carried out at room temperature, power–frequency and 80 kV/mm field strength.
2.5 Nano-composite interface model
First, interaction zone consists of bonded region and transition region; Second, the stronger the interaction strength between nanoparticles and polymer matrix, the thicker the interaction zone; Third, fermi level of interaction zone is different from that of nanoparticle and polymeric matrix; Fourth, the interaction zone is a low density region as well as a less stoichiometric region, leading to the increase of free volume and the formation of traps; Fifth, the interaction zone is a conductive region compared with the polymeric matrix.
3 Results and discussion
3.1 Characterization of Si-B nanoparticles
3.2 Dielectric properties and the Cole–Cole plot
In the full frequency range, the dielectric loss of nano-doped composites is lower or equal to that of pure EP materials, but in the high frequency region, the difference of dielectric loss between samples with different doping amounts becomes larger. The reason is that number of the interface increases due to the finite doping amount. However, the interaction between dipole on nanoparticles and dipole on macromolecular matrix will increase to a certain extent. In the low frequency region, the increase of interfacial polarization and the corresponding dielectric loss between composites with different doping amounts are limited. Moreover, the polarization of space charge nearby will be restricted by the transition region, which reduces the interface polarization, resulting in the loss reducing caused by interface polarization . In high frequency range, the inhibiting effect of transition zone on dipole polarization becomes more and more obvious with the increase of frequency, which indicates that the difference of dielectric loss become large between composite samples with different doping content.
3.3 The space charge limited current
In terms of the theory of space charge limiting current, in low electric field, the electric current in the polymer follows Ohm’s law. When the electric field exceeds a certain field strength, the number of carriers will increase because of de-trapping of the trapped space charges, and the current increase. The volt-ampere curve shows a turning point, and the voltage of turning point is the aging threshold. When the voltage exceeds the aging threshold, the current goes into the space charge limited current area. In the space charge limited current area, the trap structure of the polymers plays an important role to the current. The slope of the curve in the space charge limited current region mainly reflects the de-trapping velocity of carrier. The larger the slope is, the faster the space charge de-trapping rate is .
The Si-B oxide nanoparticles are dispersed in EP and form a large number of carrier traps in the organic/inorganic interface. The organic/inorganic interface increase with the increase of nanoparticle content. The greater the probability of interaction between the dispersed phases of nanoparticles, the greater the probability of producing deep traps . When the carrier is trapped by a deep trap, it is difficult to jump out the trap and easy to form the space charge electric field. The more the doping amount, the stronger the space charge electric field. Besides, the space charge electric field will be more continuous with the more uniform dispersion. The space charge electric field will limit the migration of carriers and reduce the current slope in the space charge limited current region. In addition, the effective value of the external electric field will be more weakened as the increase of doping amount, which makes the de-trapping rate of carriers slow down.
3.4 The breakdown strength and Corona resistance
There are considerable carrier traps in the interface between Si-B nano-oxide particles and EP. They capture carriers and form space charge electric field which will hindere the migration of charged particles produced by air ionization . Therefore, the particles not only have excellent corona resistance, but also have shielding effect to protect EP matrix due to the formation of space charge electric field, which make the corona corrosion rate of EP matrix slow down. In addition, even when nano-oxide particles are exposed on the surface of polymer, oxides protective layer will be formed, which can prevent charged particles from bombarding the polymer matrix. The air ionization around electrodes under corona action will produce ozone and other gases, which will cause the oxidative degradation of polymer matrix. Because of the barrier effect of nano-oxide protective layer, the oxidative degradation will be prevented. Moreover, the nanoparticles dispersed in the polymer can also prolong the path of carrier migration in the composites, that is to say, reduce the carrier mobility. All these are the reasons for the improvement of corona resistant life of Si-B/EP composites.
Figure 13 shows that the breakdown field strength of Si-B/EP composites begins to decrease when the doping content of Si-B exceeds 5 wt%. When the doping content reaches 7 wt%, the breakdown field strength of Si-B/EP composites decreases to 28.77 kV/mm. That is to say, in this experiment, the breakdown field strength of composites has a significant impact on the corona resistant life. With the decrease of breakdown field strength, the corona resistance life of composites is shortened.
In this paper, Si-B oxides were designed and synthesized. The Si-B/epoxy nano-composites have been investigated. The dielectric constant and dielectric loss of Si-B/epoxy nano-composites decrease gradually with the increase of Si-B nanoparticles content. It is interesting that the conductance does not change significantly with the increase of doping amount. The dielectric relaxation behavior of composites was studied by analyzing the Cole Davidson arc. The asymmetric distribution of relaxation time becomes wider with the increase of filler loading. As the dipole interaction in the interface region widened the relaxation time distribution. The results indicate that the interaction zone around Si-B nano-particles plays an important role in the dielectric constant, dielectric loss and conductance. With the increase of filler loading, the trap density in the composite increases and the interaction between traps becomes larger which reduces the de-trapping energy and the electric aging threshold. Meanwhile, the concentration of deep traps increases. The repulsion effect of space charge electric field becomes stronger, and the protective effect of nanoparticles on EP matrix becomes better. However, due to the decrease of dielectric strength, the corona resistance would not been further improved. When the doping content was 6 wt%, the maximum value was 17.6 h, which was 8.38 times that of pure EP material.
This work was financially supported by the Fundamental Research Fundation for Universities of Heilongjiang Province (LGYC2018JC033), the National Natural Science Foundation of China (No. 51277044).
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