https://doi.org/10.1007/s13369-020-04686-1
RESEARCH ARTICLE-ELECTRICAL ENGINEERING
Effects of Compression and Silica Addition on the Dielectric Properties of Epoxy Composites
Abraiz Khattak1 · Kashif Imran1 · Asghar Ali1 · Zuhair Subhani Khan1 · Abasin Ulasyar1 · Muhammad Amin2 · Adam Khan3,4 · Azhar Ul Haq5
Received: 19 October 2019 / Accepted: 30 May 2020
© King Fahd University of Petroleum & Minerals 2020
Abstract
Composite dielectrics and insulators are emerging as replacement of neat epoxy because of their superior characteristics.
However, methods of composite preparation, type and size of filler and stresses such as temperature and mechanical stresses influence these characteristics. In addition, inorganic oxides-based epoxy composites are expected to have good dielectric characteristics. In this work, effect of compression and silica on the dielectric properties of epoxy composites with refer- ence to neat epoxy is explored. Epoxy microcomposite (15% by weight silica loading) and nanocomposites (2.5% and 5%
by silica loadings) are prepared, and their dielectric properties are investigated before and after compression at 15 MPa.
Overall epoxy nanocomposites with 5% nanosilica performed best having 0.09 dissipation factor and dielectric constant of 6.23. After compressing the samples at 70 °C and 15 MPa, dramatic increase in the dissipation factor of microcomposites was recorded where the average 𝜅 increased by a factor of 1.53–8.27 in comparison with 5.40 in the uncompressed form.
Keywords High voltage · Dielectric properties · Insulation · Dissipation factor · Epoxy · Composites
1 Introduction
Polymeric dielectric and insulators are gaining attention due to their several advantages over conventional ceramics [1–4]. They are not only used for general electrical isolation [5, 6] but also possess good characteristics as high-voltage insulators [7, 8] and dielectrics for energy storage applica- tions [9, 10]. Among several types of polymers, epoxy is
one of the best choices in specific applications where hard and fire resistive polymers with high mechanical endur- ance are required [11–14]. Epoxy is not only known for utmost mechanical endurance but also possesses excellent dielectric properties [15, 16]. Advancements in the field of nanotechnology are facilitating enhancements in the dielec- tric and mechanical characteristics of polymers. Recently, composites based on diverse inorganic oxide fillers such as alumina [17], zinc oxide [18] and titania [19] are reported with enhanced dielectric properties as compared to neat epoxy. In the case of composites, better Young’s modulus and hardness with improved thermal stability and conduc- tivity were achieved [20]. In another study [21], the weight loss as function of temperature was slower in composites in comparison with the unfilled epoxy. In some studies, nano- and microfillers were also proved to have great impact on the aging behavior of polymers [22–25]. However, the dielec- tric properties are of utmost importance and require further exploration for electrical insulation and energy storage appli- cations [26, 27]. Although improvement in dielectric and other properties of composites was attained to a great extent, yet several stresses such as mechanical and environmental stresses [28–30] affect these characteristics.
* Adam Khan
1 U.S.-Pakistan Center for Advanced Studies in Energy, National University of Sciences and Technology (NUST), Sector H-12, Islamabad 44000, Pakistan
2 Department of Electrical Engineering, Ghulam Ishaq Khan Institute (GIKI) of Engineering Sciences and Technology, Topi, Pakistan
3 Informetrics Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam
4 Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam
5 Department of Electrical Engineering, College of Electrical and Mechanical Engineering (CEME), National University of Sciences and Technology (NUST), Rawalpindi, Pakistan
Among several groups of fillers, inorganic fillers are the most suitable choice for dielectric and insulation applica- tions. They have high band gap, high resistivity, high die- lectric strength and overall good insulating nature. Among inorganic types of fillers, silica showed promising results for thermal properties, mechanical features, aging behavior and resistance against outdoor stresses [31]. By comparison, some of the studies reported effect of mechanical pressure and compression on the electrical characteristics of poly- mers. This is equally critical for epoxy because mechani- cal stresses in electrical insulation applications may cause combined through-thickness compression with inter-laminar shear [32] that affects dielectric properties. Furthermore, through-thickness variation and interfacial polarization of air plasma may have significant impact on the dielectric nature of the epoxy [33–35]. In a study in [36], significant variation in electrical conductivity was observed after com- pression. Authors in [37] also concluded that the dielec- tric properties vary as function of mechanical pressure and this variation was attributed to pressure-induced tetragonal orthorhombic-cubic structural transitions. The dielectric properties of epoxy–silica composites and their depend- ency on mechanical compression have not been studied so far. This is momentous for the steps toward replacement of neat epoxy with epoxy–silica composites in dielectric and electrical insulation applications.
Based on the above-mentioned motivations, this work is dedicated to analyze dielectric properties and their depend- ence on compression for different epoxy–silica nano- and microcomposites.
2 Experimental
2.1 Preparation of Nanocomposites
Resin diglycidyl ether of bisphenol-A (DGEBA) (Eposchon®), equivalent to 189 ± 5 g/eq of epoxy, and the hardener cycloaliphatic amine (EPH 555®) equivalent to 86 g/eq of amine hydrogen were used. Both resin and hardener were obtained from Justus Kimia Raya, Indone- sia. Nanosilica (AEROSIL® 200) with average diameter of 12 nm and specific surface area of 200 m2/g was obtained from Degussa, USA. Microsilica was used with average par- ticle size of 5 µm, obtained from Wuhan NewReach chemi- cals China. Silane (DOW CORNING® Z-6040) was used for surface functionalization of fillers, while ethanol used as solvent was procured from Sigma-Aldrich®, USA).
Silica was poured and dispersed in ethanol to avoid stick- ing. Solution was kept in an ultrasonic bath for 90 min to get maximum dispersion. Then silane was mixed for surface functionalization of the silica. In the next step, a high shear mixer of rotational speed 3600 rpm was used for mixing of
epoxy resin with the mixture. After that, the solution was again shifted to the ultrasonic bath. Thereafter, ethanol- based solution of epoxy and silica was subjected to vacuum at temperature above the boiling point of the ethanol. For complete evaporation of ethanol, the solution was placed in the vacuum oven for 48 h. In the next step, the hardener was added in solution and placed in a mixer with rotational speed of 7200 rpm for 20 min before moving the mixture to the ultrasonic bath. The solution was then kept in vacuum at 27 in-Hg until the bubbles were completely removed. After that, the solution was transferred into casts and placed at 25 °C for 1 day. Finally, the prepared samples were post-cured in a hot oven for few hours. A list of samples prepared by above described procedure is presented in Table 1.
2.2 Measurements and Methods 2.2.1 X‑ray Diffraction
An X-ray diffractometer of D8 ADVANCE DAVINCI design by Bruker was used for confirmation of composition, struc- ture and distribution Cu Kα radiations with LYNXEYE 1-D detector and 2θ angle range of 5°–85° which were used for the analysis.
2.2.2 Dielectric Properties Measurement
For the dielectric measurements, 7600 Plus LCR meter along with a rigid dielectric cell LD-3 from IET Labs, USA, was employed. For all the samples, the diameter was kept approximately 7.5 cm. The measurements were carried out in the range of 10 Hz to 2 MHz. For comparative analysis, 56 kHz–2.0 MHz range was selected. This procedure was applied both before and after compression.
The dielectric constant ( 𝜅 ) is the ratio of the permittivity of a material ( 𝜀m ) to the permittivity of free space or air ( 𝜀o ), as given below.
The capacitance ( C ) of a parallel plate capacitor may be calculated as
(1) 𝜅 =𝜀r = 𝜀m
𝜀o
Table 1 Prepared epoxy composite samples
Sample Short name
Neat epoxy Neat
Epoxy-2.5 wt% nanosilica composite ENC 2.5
Epoxy-5 wt% nanosilica composite ENC 5
Epoxy-15 wt% microsilica composite EMC 15
where ‘A’ is the plate area separated by a dielectric of per- mittivity ( 𝜀 ) by a distance d.
Therefore, 𝜅 of a dielectric material may also be expressed as the ratio of the capacitance of the parallel plate capacitor with solid dielectric medium ( Cm ) to the capacitance of the same capacitor with air as the dielectric medium ( Co ), as given below.
The above equation was used to calculate 𝜅 of the epoxy–silica systems.
2.2.3 Compression Conditions
For compression, the epoxy–silica systems were first heated to 70 °C and then compressed by applying 15 MPa pres- sure. Both the heating and compression were carried out with a mounting press CY-600D, China. After compression, the samples were de-shaped and expanded. According to requirements of the LCR meter employed, the samples were cut into 7.5 cm diameter.
(2) C= A𝜀
d
(3) 𝜅=𝜀r= Cm
Co
3 Results and Discussion
3.1 X‑ray Diffraction AnalysisIn XRD pattern, neat epoxy did not show any sharp peaks, except a characteristic broad peak centered at 20.1°. This was observed in all the epoxy–silica composites. As depicted in Fig. 1, epoxy–silica microcomposite (EMC 15%) showed sharp peaks as well as the hump-shaped broad peak. The observed peaks were due to increased lattice strain [38] and due to the elastic X-ray scattering of its cured epoxy struc- ture [39]. On another note, the peak intensity was directly proportional to the epoxy content. Figure 1b displays a com- parison of the peak intensities. The peak intensity was maxi- mum for the neat sample, followed by ENC 2.5 and ENC 5, respectively, whereas the peak intensity was minimum for the EMC-15. This observation was in line with the percent- age wt. loading of nanosilica. Thus, the sample with the least silica content exhibited the strongest hump, while the EMC- 15 having highest silica content showed weakest hump. It was also worth noticing that nanocomposites had significant silica content, but no peculiar silica peak was visible in the XRD patterns due to amorphous nature of nanosilica [40].
Conversely, the microsilica used was crystalline in nature.
The epoxy–silica microcomposite, EMC 15, showed some sharp peaks at 17.7°, 22.6° and 28.2° as well as several minor peaks superimposed on the amorphous epoxy pattern.
The peak positions and relative intensities closely matched with that of SiO2 phases such as quartz low, quartz, silica X and synthetic quartz with PDF# 86-1565, 79-1913, 34-1382 and 33-1161, respectively.
Fig. 1 X-ray diffraction patterns of neat epoxy (Neat), epoxy-2.5% SiO2 nanocomposite (ENC-2.5), epoxy-5% SiO2 nanocomposite (ENC 5), and epoxy-15% SiO2 microcomposite a relative permittivity, b relative intensity
The shift in some of peak positions in phases mentioned above indicated that the SiO2 was not in a fully relaxed form but rather existed in strained condition. This strain was due to the silane functionalization of SiO2 particles.
After functionalization, surface functional groups such as the hydroxyl (–OH) group in silanol (Si–O–H) pulled out the atomic layers to which they were attached and that resulted in increased interlayer spacing and relatively higher strained structure. This was evidence of successful functionalization of silica before preparation of composites. The reason for using functionalized silica was to enhance the organic–inor- ganic interactions for improved filler dispersion, increased epoxy reactivity, better adhesion and enhanced composite strength [41, 42].
3.2 Results at Low Frequency
Energy loss occurs in capacitors, and a practical capaci- tor can be considered as an ideal capacitor connected with equivalent series resistance ( ESR ). The ESR presence in a capacitor may be attributed to electronic conduction and dipole relaxation in the dielectric medium. For a capacitor, dissipation factor ( DF ) is used to express its purity and it is the ratio of the real power loss in the ESR to the reactive power oscillating across its capacitive reactance (XC).
Mathematically,
For a sinusoidal AC supply of frequency f
Thus,
Putting equation Eqs. (4) and (5) together yields
According to Eq. 7, when a dielectric of known ESR is subjected to a sinusoidal supply of frequency f , the dissipa- tion factor changes as function of k. Therefore, for dielec- trics the higher is the dielectric constant (𝜅) the higher is the energy storage capability. However, for insulation pur- poses, lower 𝜅 and lower dissipation factor DF are required.
Measurement of an accurate value of actual resistance of the capacitor in the low and very high frequency ranges is quite challenging. This is due to high impedance at low frequency and high displacement current at high frequencies. In case of low-test frequencies, the LCR meter cannot determine the correct phase angle as the impedance exceeds a certain (4) DF= ESR
||XC|
|
(5)
||XC|
|= 1
2𝜋fCm
(6) DF=ESR(
2𝜋fCm
)
(7) DF=(
2𝜋f ESRCo) 𝜅
threshold value and thus ESR value at these frequencies is not reliable. Similarly, at high frequencies the high displace- ment current is due to small series resistance. Thus, resist- ance measured at these frequencies may not be equal to the DC/actual resistance of the dielectric. In the current case, the required impedance was in the range of 100 Ω to 100 kΩ for best measurement accuracy. Therefore, measurements were expected to be more accurate at relatively higher frequen- cies, whereas the results obtained at lower frequencies were misleading. An example of such erroneous results (at lower frequencies) between 10 Hz and 2 MHz range is presented in Fig. 2. The term ESR
n is the normalized value equivalent to the ratio of the ESR at a particular to the maximum ESR value recorded for a sample.
For frequencies below 20 kHz, a sudden increase in ESR as a function of the test frequency was recorded. This abrupt n
increase may be correlated to the impedance that increased beyond the 100-kΩ limit. Since the LCR meter cannot accu- rately determine the phase angle for such high impedances, both the ESR and XC may not be relied upon. Such effects become even more pronounced in case of DF’s measure- ment. Therefore, it is logical to consider the DF in the range where the readings are more accurate. Consequently, 𝜅 and DF were analyzed between the 56 kHz and 2 MHz range.
3.3 Comparative Analysis in 56 kHz–2.0 MHz Range The effects of silica addition on the 𝜅 of epoxy are depicted in Fig. 3. For all the samples, frequency dependency of the
Fig. 2 ESR
n is the equivalent series resistance ( ESR ) at the test fre- quency normalized to the maximum ESR value at the lowest test fre- quency of 10 Hz and plotted against the test frequency. The sudden increase in the ESR
n relative to the frequency is a systematic error, for the LCR meter cannot measure the correct phase angle at high impedance (i.e. low frequency) values. Otherwise the ESR
n is almost constant for all the samples beyond the 5 kHz frequency
𝜅 was observed. The 𝜅 of neat epoxy was lowest, with an average value of 5.15. It was followed by EMC-15 with an average value of 5.4, whereas ENC 2.5 exhibited greater average value of 5.53. In comparison, ENC-5 showed 6.23 as the average 𝜅 value that was highest among all the samples.
As a general trend, value of 𝜅 increased proportionally to the SiO2 filler content. However, EMC-15 having the highest concentration of 15% wt. percentage SiO2, showed relatively irregular response. This could be due to reduced surface area and thus reduced interactions with the surrounding epoxy.
From the above results, it can be inferred that the addition of silica into the epoxy structure can enhance the overall 𝜅 of the system. However, this impact was more pronounced in nanosilica in comparison with microsilica. This feature of the epoxy–SiO2 system may be attributed to the uniform distribution and overall increased surface area of the nano- particles in the composite. Figure 4 graphically compares the quality of the neat and composites capacitive systems in terms of the dissipation factor and shows that DF increased with the increase in the test frequency. The reason was a significant decrease in the displacement current for the lower frequencies. This is also in line with Eq. (7). Similarly, increase in 𝜅 was recorded with the decrease in DF . The neat epoxy exhibited the least dissipation factor with an average value of 0.06, followed by EMC-15 with average value of 0.08. Moreover, in case of both ENC-2.5 and ENC-5 almost the same average DF value of 0.09 was observed.
Since silica possesses tremendous insulating proper- ties, the ESR for ENC-5 was comparable to that of ENC 2.5. Therefore, even XC increased in conformance with the 𝜅 , but the higher ESR of ENC 5 neutralized its XC incre- mented to a level that the DF did not increase significantly.
In consequence, both ENC 2.5 and ENC-5 showed identi- cal DF values for the whole frequency range. The behavior of EMC-15 was inconsistent because of limited molecular level interaction. Non-uniform distribution of microfiller causing significant amount of voids with trapped air maybe another reason for the inconsistent behavior of EMC-15.
Such behavior is in full conformity with its 𝜅 behavior represented by Eq. (4).
One of the main reasons for superior dielectric behav- ior of epoxy nanocomposite at 5% loading, in compari- son with other samples, is occurrence of abundant silanol groups at its surface. Silanol provides excellent hydrogen binding with the polymer and thus provides stability to overall structure. This particle–polymer binding at interac- tion zone increased overall inertness and intactness of the structure [43]. The same improvement was not achieved, even at 15% loading of microsilica, due to more than 9 times increased silanol groups on nanosilica surface. Con- sequently, the increased silanol groups led to increased probability of polymer–filler interaction. This elucidates the reason that nanofiller is required in low amounts as compared to microsilica to accomplish the same enhance- ment in characteristics of polymer composites [44].
Secondly, at 5% wt. loading the nearest neighbor index is approximately 1, which indicates highest dispersion.
While more than 5% concentrations may cause agglomer- ate, clustering and increased filler–filler attractions, such concentrations cause loss in certain characteristics of pol- ymers [45]. In contrast, microfiller loadings can exceed even above 40% concentrations. However, loss in elemen- tary features, such as thermal or dielectric characteristics, may occur at very high loadings [46]. Therefore, to avoid filler–filler interaction microfiller is used at 10–20% by wt. loading [47, 48].
Fig. 3 Dielectric constant of epoxy with different concentrations and micro- and nanosilica as filler material in the frequency range of 56 kHz to 2 MHz
Fig. 4 Dissipation factor of the prepared samples
3.4 Compression Effect
In an effort to analyze the electromechanical properties of the epoxy–SiO2 system, the samples were compressed at 15 MPa and 70 °C. The effects of compression on the 𝜅 are presented in Figs. 5, 6 and Table 2.
For the neat epoxy, the average value improved 1.06 times of uncompressed sample i.e. 5.44. Similarly, enhancement in 𝜅 was also recorded in ENC 2.5 and reached average value of 5.74 in comparison with 5.53 of the uncompressed sam- ple. For ENC 5, the average 𝜅 was increased by a factor of 1.12–6.96 relative to 6.23 of the uncompressed sample.
The most pronounced effects were observed for EMC 15, where the average 𝜅 increased by a factor of 1.53–8.27 in comparison with 5.40 of the uncompressed sample. This can be attributed to the removal of voids and compactness achieved after compression. Figure 7 shows dissipation
factor before compression while Fig. 8 and Table 3 are given for comparison of the after-compression effects on the DF of epoxy–SiO2 samples. After compression, a trend similar to that of the 𝜅 was observed. The DF was least for the neat epoxy with an average value of 0.07. For the neat epoxy, the DF increased by a factor of 1.17 after compression. Dissipa- tion factor of the ENC 2.5 was even more and increased by a factor of 1.11 after compression from the uncompressed value of 0.09–0.10. For ENC-5, the DF was maximum aver- aging at 0.12 compared to 0.09 of the uncompressed sample.
Therefore, the DF increased by a factor of 1.33 for ENC-5. In comparison, increase in DF of only 1.14 times was observed for EMC. As evident, the DF of EMC-15 did not rise that much as expected in correlation to its 𝜅 after compression.
This is because DF is a ratio of the ESR to XC , whereas XC is directly proportional to the magnitude of the displace- ment current and ESR may be related to the current in phase
Fig. 5 One-on-one comparison of a Neat epoxy, b epoxy-2.5% SiO2 nanocomposite. c epoxy-5% SiO2 nanocomposite and d epoxy-15% SiO2 microcomposite, before (-uncompressed) and after (-compressed) thermal treatment at 70 °C and compression at 15 MPa pressure
with the applied potential. It was observed that the ESR of the EMC was relatively higher than rest of the epoxy–silica composites and it was probably because of the filler–filler interaction that resulted in air voids.
Overall, the results obtained were in conformance with the related previous work. For example, Yu et al. [49]
obtained improved dielectric strength and permittivity for nanoalumina-based epoxy nanocomposites. Singha et al.
[50] studied ZnO-based epoxy composites and signifi- cant increase in the dielectric strength, and loss factor was reported. In another study by Cheng et al. [51], epoxy–silica nanocomposites were studied and the characteristics were reported as function of surface characteristics of nanofill- ers. In a study, Santanu Singha et al. also reported tan delta and permittivity of TiO2 and ZnO-based nanocomposites to conclude that for the dielectric properties of nanocompos- ites, interfacial characteristics are of utmost importance in comparison with other parameters. Overall, nanocompos- ites based on inorganic oxides fillers other than silica were reported to have improved dielectric characteristics over neat
epoxy and in agreement with the presented work. However, detailed review by Singha et al. [52] may further strengthen the understanding of improved dielectric properties and compression effects on inorganic oxides-based dielectric nanocomposites.
4 Conclusions
Silica-based epoxy micro- and nanocomposites were pre- pared, and compression effect on the dielectric properties was analyzed. During initial investigation, the results at lower frequencies were found invalid due to machine limi- tations. Therefore, all comparisons were performed above 56 kHz range as required for available instrument. Overall epoxy nanocomposite at 5% silica loading showed highest dielectric properties having average DF value of 0.09 and dielectric constant of 6.23. After compressing the sam- ples with 15 MPa pressure at 70 °C, the average DF value recorded was 0.12 and dielectric constant increased to 6.96.
Fig. 6 Dielectric constant of epoxy with different concentrations and micro- and nanosilica after heat treatment at 70 °C and compression up to 15 MPa in the frequency range of 56 kHz to 2 MHz
Table 2 Average dielectric constant (𝜅) in the 56 kHz to 2 MHz range and its ratio for the compressed sample to that of the uncompressed sam- ple
Sample nomencla-
ture Neat epoxy ENC-2.5 ENC-5 EMC-15
Compressed/uncom-
pressed Uncompressed Compressed Uncompressed Compressed Uncompressed Compressed Uncompressed Compressed
Average 𝜅 5.15 5.44 5.53 5.74 6.23 6.96 5.4 8.27
Average 𝜅 ratio (compressed/
uncompressed)
1.06 1.04 1.12 1.53
Fig. 7 Dissipation factor of the compressed samples
In case of microcomposites, a higher increase of 1.53 times in the dielectric constant was recorded. This was attributed to air voids formed due to large size of microsilica before the compression. However, a marginal increase of only 14% was observed in average dissipation factor of microcomposite,
whereas nanocomposites having 5% nanosilica showed highest increase of 33%. Three important conclusions can be drawn from these findings. First, the accuracy of dielec- tric measurements is highly dependent on impedance of the dielectric. Therefore, for comparative analysis complete
Fig. 8 One-on-one comparison of the as obtained uncompressed and compressed samples of a neat epoxy, b epoxy-2.5% SiO2 nanocomposite, c epoxy-5% SiO2 nanocomposite and d epoxy-15% SiO2 microcomposite
Table 3 Average dissipation factor (DF) in the 56 kHz to 2 MHz range and its ratio for the compressed sample to that of the uncompressed sam- ple
Samples’ nomen-
clature Neat EMC ENC-5 EMC-15
Sample nomencla-
ture Uncompressed Compressed Uncompressed Compressed Uncompressed Compressed Uncompressed Compressed
Average DF 0.06 0.07 0.09 0.10 0.09 0.12 0.07 0.08
Average DF ratio (compressed/
uncompressed)
1.17 1.11 1.33 1.14
frequency range of instrument can be misleading. This is due to high ESR at low frequency and high displacement cur- rent at high frequencies. Therefore, a suitable range where the LCR meter can accurately determine the correct phase angle should be selected. Secondly, silica-based composite dielectrics possess superior properties in comparison with neat epoxy. However, this improvement is different for dif- ferent composites. Due to several folds increase in surface area and interaction at nanolevel, the nanocomposites have better dielectric performance than microcomposites. Third, the compression has great impact on the dielectric proper- ties of epoxy micro- and nanocomposites. Besides gener- ally better mechanical characteristics of microcomposites, the nanocomposite with 5% nanosilica sustains its dielec- tric properties and even improves its dissipation factor after compression.
Acknowledgements No external funding was received for this project.
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