Epoxy Alumina Nanocomposites Research Articles

Analysis of Selected Dielectric Properties of Epoxy-Alumina Nanocomposites Cured at Stepwise Increasing Temperatures

The paper presents the effects of gradual temperature curing on the dielectric properties of epoxy nanocomposite samples. Samples were prepared based on Class H epoxy resin filled with nano-alumina (Al2O3) for different wt% loadings (0.5 wt% to 5.0 wt%) and two different filler sizes (13 nm and <50 nm), i.e., two different specific surface area values. During the research, specimen sets were cured gradually at increasingly higher temperatures (from 60 °C to 180 °C). Broadband dielectric spectroscopy (BDS) was used to determine the characteristics of the dielectric constant and the dielectric loss factor in the frequency range from 10−3 Hz to 105 Hz. As a result, it was possible to analyze the impact of the progressing polymer structure thermosetting processes on the observed dielectric parameters of the samples. The nano-Al2O3 addition with 0.5 wt%, 1.0 wt%, and 3.0 wt% resulted in a decrease in dielectric constant values compared to neat epoxy resin samples. The most significant reductions were recorded for samples filled with 0.5 wt% of 13 nm and <50 nm powders, by about 15% and 11%, respectively. For all tested samples, the curing process at a gradually higher temperature caused a slight decrease in the dielectric constant (approx. 2% to 9%) in the whole frequency range. Depending on the nanofiller content and the curing stage, the dielectric loss factor of the nanocomposite may be lower or higher than that of the neat resin. For all tested samples cured at 130 °C (and post-cured at 180 °C), the differences in the dielectric loss factor characteristics for frequencies greater than 100 Hz are low. For frequencies < 100 Hz, there are prominent differences in the characteristics related to the size of the nanoparticle and the individual wt% value. At a small nanofiller amount (0.5 wt%), a decrease in the dielectric constant and dielectric loss factor was observed for frequencies < 100 Hz for samples with nanofillers of both sizes.

Probing thermal conductivity of interphase in epoxy alumina nanocomposites

The objective of this research is to determine the thermal conductivity of the interphase in epoxy alumina nanocomposites. First, TPS 500 measures the thermal conductivity of epoxy alumina nanocomposite samples. Following that, a numerical model based on the finite element method was developed to estimate the effective thermal conductivity of epoxy alumina nanocomposites over a range of assumed interphase thermal conductivity values. Finally, an algorithm is devised to extract the interphase’s thermal conductivity by combining simulation and experiment results. Interphase was found to have significantly higher thermal conductivity than the base polymer. A comprehensive analysis is presented to shed light on the observed increase in interphase thermal conductivity. The findings of this study will be critical for further investigation of heat transfer in epoxy alumina nanocomposites via modeling and simulation studies.

Influence of Nanofiller Dispersion on Electrical and Mechanical Properties of Epoxy Alumina Nanocomposites

The electrical and mechanical properties of the epoxy alumina nanocomposites depend on the uniform dispersion of the nanofiller in the epoxy matrix. Epoxy alumina nanocomposites were prepared using 1, 3, and 5 wt% of alumina nanofiller, and electrical and mechanical properties were analyzed using experimental and modelling studies. Water droplet initiated corona inception voltage (CIV) was identified using fluorescence fiber technique and by Ultra High Frequency (UHF) technique, under AC and DC voltages. The CIV formed due to water droplet have reduced drastically with increase in number of droplets and fluorescent fiber technique found to be more sensitive to identify water droplet initiated discharges. A micro mechanical model was proposed to analyse the combined effect of the interphase and agglomeration properties of the alumina nanoparticles on the tensile strength of epoxy alumina nanocomposites. Variation in ϕagg, Eagg, and E were analysed by adopting the non-parametric distribution of alumina nanoparticles and Young’s modulus increased with the increment in the alumina nanofiller dispersion level. The presence of aggregated particles exhibits a negative effect on the tensile properties of nanocomposites.

Use of LIBS technique for identification of type of pollutant and ESDD level on epoxy-alumina nanocomposites using ANN

Epoxy-alumina nanocomposites have been coated with different types of pollutants at various concentrations in order to analyze their pollution performance using laser induced breakdown spectroscopy (LIBS) study. Elemental analysis of spectral peaks has been successful in determining the elemental composition of the pollutant present on the surface of the specimens. The conductivity and equivalent salt deposition density (ESDD) values have been determined for all the pollutants at different concentrations. A direct correlation between ESDD and the normalized intensity ratio of LIBS spectral data is noticed. Regression coefficient (R 2) has been used as a performance parameter in correlating the ESDD and normalized intensity ratio. Artificial neural network (ANN) technique has been adopted to LIBS spectral data for the classification of the pure epoxy and 3 wt% epoxy-alumina nanocomposite specimen based on conductivity and type of pollutant. ANN developed with conjugate gradient backpropagation with Polak-Ribiere updates training algorithm has reflected higher classification accuracy and required lesser epochs to converge, compared to other training algorithms in classifying the contaminated specimens.

Sensing the Thermal Aging of Epoxy Alumina Nano-Composites Using Electric Modulus

In this contribution, a novel technique to accurately sense the thermal aging of the epoxy alumina nano-composites employing complex electric modulus is proposed. To sense the thermal ageing of epoxy nano-composites accurately, knowledge of relaxation behavior is necessary. However, relaxation behavior cannot be understood completely due to electrode polarization and charge transport effects at high temperatures and at low frequencies. To overcome this problem, electric modulus which is defined as the inverse of the complex permittivity, is being proposed in this study to analyze the aging behavior of the epoxy alumina nano-composites quantitatively. For this purpose, three epoxy resin samples mixed with alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) nano-fillers with different filler concentrations were prepared and thermal aging of the same samples was done for 100 hours, 200 hours, 300 hours and 400 hours, respectively. For each sample, the complex dielectric modulus (M*( ω)) was computed using frequency domain spectroscopy measurement to observe their frequency dependent relaxation behaviors. The variation of the real (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> ( ω)) and imaginary (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> ( ω)) part of ( M*( ω)) over the frequency range from 1 mHz to 10 kHz was further fitted using Cole-Cole (C-C) model. From the nature of variation of M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> ( ω) spectrum and the fitting parameters of the C-C model, two characteristic parameters were extracted to quantitatively describe the thermal aging of the epoxy nano-composite samples. Investigations revealed that the extracted parameters can be accurately used to sense the aging condition of the epoxy nano-composites.

Understanding the impact of space charge variations with UV‐ and water‐aged epoxy alumina nanocomposites adopting pulsed electroacoustic techniques

Epoxy alumina nanocomposites were prepared to understand the space charge variations with ultra violet (UV)- and water-aged specimens. Addition of alumina nanofiller to epoxy resin had suppressed the space charge formation. A pulsed electroacoustic technique was employed to measure the space charge accumulation in epoxy–alumina nanocomposites. A marginal increase in space charge with enhanced electric field in the bulk volume of insulation was observed with UV- and water-aged nanocomposites. It was observed that 3 wt% sample showed better performance in all ageing conditions, which was confirmed through space charge studies and by the measurement of the dielectric parameters especially dielectric constant and tan(δ).

Fatigue behavior of epoxy alumina nanocomposite – role of particle morphology

In this work, epoxy alumina nanocomposites were synthesized with Al2O3 nanofiller by instant cross-linking process. Two different particle morphologies (viz. rod and spherical shape) were selected as filler to investigate their effect along with wt.% (0.5, 1.0, and 1.5 wt%) on the fatigue behavior of epoxy alumina nanocomposites. Stress to number of cycles to failure (S-N curve) as per ASTM D 7791 and fatigue crack growth (FCG) rate approach as per ASTM E 647 were used to evaluate fatigue life of epoxy alumina nanocomposites. Transmission and scanning electron microscopy were used to characterize dispersion and fracture surfaces of epoxy alumina nanocomposites. Particle morphology and wt.% influenced the fatigue life and FCG behavior of epoxy composite appreciably. Addition of spherical Al2O3 nanoparticles substantially improved the fatigue life of nanocomposites than unfilled epoxy at all levels of applied stresses. Fatigue life decreased with increase in wt.% of spherical Al2O3 nanoparticles and opposite trends is witnessed for nanorod filler. Higher resistance to FCG provided better fatigue life of spherical Al2O3 epoxy composites than nanorods and unfilled epoxy. Crack bowing and bifurcation, yielding, deep craters were the main fracture micro mechanism for high fatigue life of spherical Al2O3 nanoparticles reinforced epoxy composites. Various parameters like Young’s modulus, fatigue strength coefficient and exponent, Paris constants and strain energy were used to describe the fatigue and fatigue crack growth behavior.

Role of filler morphology on friction and dry sliding wear behavior of epoxy alumina nanocomposites

Epoxy alumina composite was fabricated by instant cross-linking process containing 0.5, 1.0 and 1.5 wt.% of alumina reinforcement. Ultrasonication process is used to disperse alumina nanoparticles in epoxy. To study effect of particle morphology two types of alumina nanoparticle’s were used, one was of rod shape whose diameter varies from 5–10 nm and length not more than 50 nm. Whereas other type of particle used was spherical in shape with diameter varying from 27-43 nm. Reinforcement of alumina nano particles enhances the micro hardness of epoxy composite. There is no significant effect of reinforcement on water absorption behavior of epoxy composite. Friction and dry sliding wear behavior was experimentally evaluated by pin on disc setup. Infusion of alumina nanoparticles improves friction and wear behavior of epoxy composite. Reduction of 34% in friction coefficient (CoF) was recorded at 1.5 wt.% of spherical alumina particles. Wear rate also decreases with by 74% at 1.0 wt.% inclusion of spherical nano particles. Calculated energy dissipation was in good correlation with wear rate, thus a good method to predict the wear characteristics of epoxy composite. Stable development of tribo-film and surface smoothening are the major phenomena in improvement of tribological behavior of epoxy composite.

Investigations on the effect of ageing on charge de‐trapping processes of epoxy–alumina nanocomposites based on isothermal relaxation current measurements

In this study, the relationship between thermal ageing and charge trapping properties of epoxy-based nanocomposites has been investigated. With ageing, any dielectric material undergoes thorough degradation. This degradation significantly affects the space charge accumulation and charge trapping behaviour of the dielectric, which are very important parameters for insulation health under high-voltage direct current (HVDC) environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of pure epoxy and epoxy alumina (Al2 O3) nano-composites at different ageing conditions. A methodology based on polarisation–depolarisation current (PDC) measurements has been proposed to identify the current component due to a dipolar relaxation in measured total IRC. This will help to identify the trap distribution characteristics more accurately compared to conventional IRC measurements. It was experimentally observed that the addition of nanoparticles significantly reduces trapped charge formation and reduces thermal degradation. It is observed that aging leads to the generation of deeper traps, while the addition of Al2 O3 nanoparticles mainly enhances the density of shallow traps. Results presented in this work indicate that epoxy–alumina nanocomposites are very much suitable in HVDC applications from the perspective of trapped charge accumulation.

Understanding the surface condition of gamma irradiated epoxy alumina nanocomposites adopting wavelets and fractal technique

The influence of alumina nanofiller and gamma irradiation on the surface potential variation of epoxy-alumina nanocomposites was investigated. The surface potential decay rate of nanocomposites has increased and the trap depth decreased with alumina nanoparticles addition to the matrix as well as upon exposure to gamma irradiation, Surface roughness was estimated using the wavelets and fractal technique. Daubechies wavelet of order 4 (db4) wavelet was chosen as the most suitable mother wavelet for surface roughness measurement. Multi resolution signal decomposition (MRSD) analysis of surface profile has revealed that with increasing wt% of alumina nanofiller in the nanocomposites, reduction in surface roughness of nanocomposites was observed. Upon gamma irradiation, the surface roughness factor at each level of MRSD has increased marginally. Fractal dimension and lacunarity were calculated for unaged and gamma ray irradiated samples and it exhibits inverse correlation.

Evidences of interphase formation and concomitant change in the dielectric properties of epoxy-alumina nanocomposites

This work unravels and sheds light on interphase formation in epoxy alumina nanocomposites. To probe interphase and its affiliation with dielectric properties, three kinds of samples are investigated viz. neat epoxy, nanocomposites (using surface treated nanofillers), and nanocomposites (with untreated nanofillers). Characteristics information received from thermogravimetric analysis and dielectric properties measurements insinuates a change in chemistry and molecular mobility at the filler matrix interfaces. Using chemical structure of different constituent phases and FTIR spectroscopic analysis a chemical interactive model is presented to elicit interphase formation in composites. The impact of interfacial interaction on long term performance of the nanodielectrics is examined by conducting endurance tests under divergent ac stress. Nanocomposites exhibit a clear superior erosion resistance over the neat polymer. Under the application of cyclic non uniform ac stress, progressive erosion is likely to initiate and grow from high stress regions. Nanofillers may act as an obstacle and force the eroded channels to move through a zig-zag path. Additionally, surface treated nanofillers owing to their strong chemical bonding with polymer matrix expected to retard damage process by alleviating fatigue and distributing electromechanical stress between filler and polymer matrix. The results of this study show compelling evidence of interphases and their prominence on dielectric properties. Moreover, this study provides an intellectual foundation to vivify how engineering the chemistry of the interfaces may help in designing nanodielectrics with enhanced functionalities.

A novel bisection method based algorithm to quantify interphase in epoxy alumina nanocomposites

Polymer nanocomposites (PNCs) are emerging materials for the future due to their superior material characteristics with respect to base polymer and composites formed with traditional micro sized fillers. Researchers are concordant in attributing the improved properties of PNCs to large proportion of polymer-filler interaction zone (or an interphase). However, literature is indeed scanty to characterize interphase in terms of its size and properties. To come into possession of fine-tuned material properties through optimal use of naofillers, characterization of interphase is necessary. In this work, we propose a novel algorithm to characterize interphase in terms of its permittivity and thickness. Epoxy resin (LY556) is used as base polymer and aluminum oxide (Al2O3, average particle size = 50 nm) is used as filler material. Filler particles are surface treated using silane. Dispersion of nano filler in polymer matrix is verified using scanning electron microscopy (SEM). Dielectric spectroscopic measurements are carried out to measure permittivity over a frequency range of 10-2 to 107 Hz. A finite element based numerical model is developed to estimate effective permittivity of composites for different values of assumed interphase parameters (i.e. thickness and permittivity). Bisection method based algorithm is devised to assign actual thickness and permittivity to interphase based on the best fit of experimental and simulated results.

Investigation of surface strain by digital image correlation and charge trap characteristics of epoxy alumina nanocomposites

Epoxy alumina nanocomposites were fabricated through shear mixing of alumina nanoparticles in to epoxy resin under optimum process conditions for its potential application as insulator in transformer. The fundamental insight in to the influence of ageing of nanocomposite insulator due to its continuous exposure to UV irradiation and water in terms of their charge trap characteristics was explored. The surface potential decay and strain variation of nanocomposite samples were measured by using Electrostatic voltmeter and Digital Image Correlation technique (DIC), respectively. Diffusion of water into epoxy alumina nanocomposite is less compared with pure epoxy resin. It has been observed that surface potential decay rate and trap depth have shown increasing and decreasing trend, respectively, with the addition of alumina nanoparticles in to epoxy resin. The UV and water aged nanocomposites exhibited a decreasing trend of surface potential decay rate and the governing reasons could be due to surface damage causing increase in deep trap formation. The tensile strength and stiffness of the samples have increased with the addition of alumina nanoparticles in to the epoxy resin. It could be due to the better bonding of alumina particles with epoxy resin. The surface strain induced in to the sample during tensile loading and the surface potential decay rate of nanocomposite has shown a direct correlation, as observed in the present work.

Investigation on space charge dynamics and mechanical properties of Epoxy Alumina nanocomposites

Epoxy nanocomposites with different wt% of alumina was prepared by use of high-speed shear mixer and high-frequency sonicator process, and the electrical and mechanical properties of the materials were investigated. Surface potential variation and trap distribution analysis were carried out with epoxy nanocomposites. Space charge analysis was carried out through Pulsed Electro Acoustic (PEA) technique. The dielectric constant and tan (δ) values of epoxy alumina nanocomposites were measured at different frequencies and temperatures. Contact angle and corona inception voltage caused by a water droplet showed a direct correlation. The viscoelastic properties of the epoxy nanocomposite material were examined through DMA studies. It was concluded that an increase in the supply frequency increases the storage modulus and loss modulus of the epoxy alumina nanocomposites. Glass transition temperature and activation energy increase with an increase in wt% of alumina content in epoxy resin. LIBS analysis indicates that the plasma temperature calculated increases with an increase in the wt% of alumina content in epoxy resin. Also, the measured plasma temperature and hardness of the material showed a direct correlation.

Rheological Properties and Curing Behaviour of Epoxy-Alumina Nanocomposites

Abstract In the present paper, effect of alumina nanoparticles on rheological properties of epoxy-alumina suspension has been investigated. Rheo-kinetic evaluation of nanocomposite was also conducted to find out the effect on gelation and vitrification behaviour of the epoxy-alumina nanocomposite. Suspension of nano particles in epoxy resin was achieved with and without the use of solvent (acetone) named as well dispersed (ultrasonication was used for dispersion of nanoparticles) and poorly dispersed (manual mixing) samples respectively. The results indicated that viscosity of epoxy-alumina nanoparticle suspension decreased, compared to that of neat epoxy at all the shear rates and wt.% of nanoparticles. Viscosity of suspension showed shear thinning behavior for alumina nanoparticles. Storage and loss moduli of suspension showed solid like behavior for both the types of dispersion with the increase in frequency. The gel time increased with the increase in wt.% of nanocomposites. The gelation of neat epoxy took less time in comparison to alumina nanoparticle

Effect of interphase on dielectric relaxation mechanisms in epoxy-alumina nanocomposite

The interphase formed between polymer and a nanoparticle plays a critical role in the dielectric properties of the nanocomposite material. The effect of interphase on dielectric relaxation mechanisms is analyzed in this work. Unfilled epoxy and epoxyalumina nanocomposite samples with 0.l, 1 and 5 wt% filler loadings are prepared. SEM analysis is conducted to ensure dispersion of nanoparticles. Impedance and permittivity measurements are taken over the frequency range of 10 mHz-10 MHz. Equivalent circuit models are derived from impedance spectroscopy measurements for unfilled and nanofilled samples. A method based on genetic algorithm is used to optimize the equivalent circuit parameters. Initialization of parameters for the algorithm is selected based on the shape of Cole-Cole plot. The proposed equivalent circuit models based on relaxation time constants indicate the presence of polarization at the interphase region in nanocomposite. Variations in configuration and parameters of equivalent circuit for unfilled and filled epoxy are used to analyze the effect of interfacial polarization. Polarization at the interphase is differentiated from other relaxation mechanisms at 0.1 and 1 wt% based on equivalent circuit model. At 5 wt%, the optimized equivalent circuit configuration and corresponding relaxation time constants different from circuit models for other filler loadings is attributed to the presence of overlapping of interphases.

Study of treeing in epoxy-alumina nanocomposites using electroluminescence

Dielectric materials in electrical equipment are mostly subjected to non-uniform or divergent field conditions. Localized high electric field may give rise to space charge which may initiate material degradation, and eventually lead to dielectric breakdown. Thus, it is important to assess the performance of nanocomposites under divergent fields before deployment as insulation. Past research has shown a positive correlation between space charge and electroluminescence (EL). In this work, EL is used as a diagnostic tool to study space charge and field-related material degradation in epoxy-alumina nanocomposites. Various experiments are undertaken to detect early stages of material degradation through EL. These include endurance tests (below and above EL emission threshold), and characterization of EL from samples pre-stressed with DC and/or AC voltages. The effect of repeated square waves on material degradation is also analyzed. Visual inspection for physical damage is carried out using a high resolution camera. The EL signature is observed to deviate significantly from its initial value at the onset of material degradation, and may therefore serve as an aging marker. In addition, the EL studies show that nanocomposites exhibit a relatively high resistance to field-assisted material degradation. The experiments on pre-stressed samples indicate that charge kinetics play a crucial role in defect initiation and material degradation.

Flexural Properties of Functionally Graded Epoxy-Alumina Polymer Nanocomposite

In this work layered functionally graded polymer nanocomposite was prepared by distribution of alumina nanoparticles (diameter in between 27 – 43 nm) in the epoxy matrix through ultrasonication. Variation in weight fraction of alumina nanoparticles is given in the thickness direction to attain the gradation. Layers of 1 mm thickness of neat epoxy, 0.25 wt.%, 0.5 wt.%, 0.75 wt.% and 1 wt.% of epoxy-alumina nanocomposites were sequentially casted in a vertical acrylic moulds to synthesize the graded composite sheet of 5 mm thickness. Tensile properties of nanocomposites for each weight fraction were determined following ASTM standard D 638-03, while the flexural properties were determined for the functionally graded polymer nanocomposite (FGPNC) by conducting three point bending test loaded from neat and 1 wt.% side. Highest increment in the tensile properties was recorded for nanocomposites at 1 wt.% of nanoparticles. An improvement of 5% in tensile strength and 14% in Young's modulus was achieved over that of neat epoxy for nanocomposites with1 wt.% of nanoparticles. In case of FGPNC, flexural strength and flexural modulus improved by 10% and 23% respectively over that of neat epoxy samples when loaded from neat epoxy side. On the other hand, when samples were loaded from 1 wt.% side, flexural strength and modulus of FGPNC showed an improvement of 6% and 17% respectively in comparison to that of layered sample of neat epoxy.

Estimation of interphase thickness of epoxy-based nanocomposites

Recent literature attributes the improvement often seen in the properties of nanocomposites to the interphase layer between the polymer matrix and the filler material. However, an understanding of the interphase in nanocomposites is still elusive. The present work aims at estimating the interphase thickness of epoxy based nanocomposites. For experimental studies, epoxy resin is used as the base polymer, and aluminium oxide (Al2O3) particles with an average size of 50 nm are used as fillers. The nanocomposites are characterized in terms of space charge density, conductivity, permittivity, and breakdown strength. All properties are measured for different nanofiller loadings (0.5 vol. % to 2 vol. %). The experimental findings reveal a correlation between interphase volume fraction and change in dielectric properties. It is assumed that the greatest change in property is achieved at that filler loading where the interphase volume fraction is maximized, provided adequate dispersion is achieved at all filler loadings. An approximate interphase thickness is estimated for epoxy alumina nanocomposites by using experimental results in conjunction with a simulation model. Accuracy of the estimated interphase thickness is seen to be dependent on the scatter in particle size and the uniformity of particle dispersion in the polymer matrix.

Life estimation of electrothermally stressed epoxy nanocomposites

Accelerated electrothermal aging tests were conducted at a constant temperature of 60°C and at different stress levels of 6 kV/mm, 7 kV/mm and 8 kV/mm on unfilled epoxy and epoxy filled with 5 wt% of nano alumina. The leakage current through the samples were continuously monitored and the variation in tan δ values with aging duration was monitored to predict the impending failure and the time to failure of the samples. It is observed that the time to failure of epoxy alumina nanocomposite samples is significantly higher as compared to the unfilled epoxy. Data from the experiments has been analyzed graphically by plotting the Weibull probability and theoretically by the linear least square regression analysis. The characteristic life obtained from the least square regression analysis has been used to plot the inverse power law curve. From the inverse power law curve, the life of the epoxy insulation with and without nanofiller loading at a stress level of 3 kV/mm, i.e. within the midrange of the design stress level of rotating machine insulation, has been obtained by extrapolation. It is observed that the life of epoxy alumina nanocomposite of 5 wt% filler loading is nine times higher than that of the unfilled epoxy.