Dynamic thermal-dielectric behavior of core-shell–structured aluminum particle-reinforced epoxy composites

Abstract

The broadband dielectric spectroscopy was carried out in the frequency range of 1–10⁷ Hz at the −20–200°C range to investigate the effect of temperature on the dynamic thermal–dielectric behavior of the aluminum (Al)/epoxy composite. The epoxy composites with core-shell–structured Al particles were prepared by solution method. The results show that the dielectric permittivity of the composites increased smoothly with a rise of filler content and reduced with an increase in frequency at room temperature. While the dielectric loss and conductivity still remained at low level owing to the nanoscale alumina insulating shell serving as a barrier layer to control the dielectric loss. The dielectric permittivity, dissipation factor, and conductivity of the composites increased with temperature and exhibited an abrupt rise around the glass transition temperature (T_g). A large increase in the dissipation factor and conductivity with temperature is attributed to the direct current conduction of thermal-activated charge carriers resulting from pure epoxy above T_g. The observed temperature-dependent dielectric relaxations of the composites indicated a thermally activated behavior of the relaxation time of epoxy chain segments.

Keywords

Polymer composite materials dielectric properties aluminum epoxy

Introduction

In recent years, polymer-based dielectric materials with a high dielectric constant (k), breakdown strength, energy density, and low dielectric loss have attracted considerable interest owing to their potential applications such as artificial muscles, energy storage, flexible electronics, embedded capacitor, and sensors.^1

–4 However, the dielectric constant of common polymers is very low (i.e. < 10).^5,6 Therefore, the key issue is to enhance the dielectric constant of polymers while retaining other excellent performances. To improve the dielectric constant, one common strategy is to add high k ceramic fillers such as barium titanate or lead titanate into the polymer matrix. Generally, the k of the composites could be obviously improved only by adding up to 50 vol% of these fillers. Unfortunately, high filler loading inevitably weakens or deteriorates the processibility, flexibility and mechanical performance of the composites.^5

–9 Another widely used strategy is to prepare percolative polymer composites by introducing high-electric conductivity particles such as metal (i.e., aurum, silver, copper, aluminum (Al))^{10

–17} and carbon fillers^{8

–23} into a polymer. Based on the percolation theory and mini-capacitor model, when the volume fraction of conductive filler increases to the percolation threshold, the dielectric constant of the composites can be dramatically enhanced.^24,25 However, these kind of composites always suffer from an abrupt variant in dielectric loss,^17
–19 imposing considerable challenge and risk in preciously controlling the percolative threshold composition to obtain reproducible products for practical applications. Therefore, the development of polymer composites with a high dielectric permittivity but low dielectric loss for applications in electronic materials has attracted ever-increasing attention and has gained remarkable progress in recent decades.^15,26
–28

Nowadays, conductor–insulator core-shell–structured particles have been employed to prepare polymeric composites with ks and low losses. The surface self-passivated Al particles are chosen as filler to fabricate the core-shell–structured Al (Al@Al₂O₃)/polymer composites with high permittivity and low dielectric loss.^{13,20,29

–34} The metal core is used to increase the dielectric constant owning to its interfacial polarization (namely, Maxwell−Wagner−Sillars (MWS) effect), while the insulator shell serves as a barrier layer to control the dielectric loss efficiently through blocking the electron transfer between adjacent metal cores.^{10,11,28,33,34} The obtained Al/polymer composites have combined the advantages of ceramics/polymer system and percolative composite, presenting a k and low dielectric loss.²⁰

In recent years, the thermal conductivity and dielectric properties of Al/epoxy composites have been investigated. However, detailed studies of dynamic dielectric behavior of Al/epoxy composites at a wide temperature range have rarely been explored. Therefore, the present article is expected to provide a deeper insight into the dynamic thermal and dielectric behavior of the Al/epoxy composites. In this study, broadband dielectric spectroscopy was employed to investigate the dynamic dielectric properties in the frequency range of 1–10⁷ Hz and at the −20–200°C range. Furthermore, the influence of temperature on the mobility and relaxation phenomena of chain segments of the Al/epoxy composite was investigated.

Experimental

Materials

A diglycidyl ether of bisphenol A-type epoxy resin (D.E.R.-331, Dow Crop, Indianapolis, Indiana, USA) with an epoxy value of 0.52–0.54 was chosen as the polymer matrix in this study. A flexible epoxy resin (long-chain polyglycol di-epoxide liquid resin, D.E.R-732, Dow Crop, Indianapolis, Indiana, USA) was also used as a reactive toughening agent to overcome the brittleness of the matrix. The curing agent was methylhexahydrophthalic anhydride supplied by Shanghai Shengyuan, China. The 2,4,6-tri(dimethylaminomethly)phenol (DMP30) (Shanghai Haitai, China) was the cure accelerator. The Al particles, with an average diameter of 1–2 µm, were purchased from Yuanyang Aluminum (Henan, China). The γ-glycidoxypropyl-trimethoxysilane, which has an epoxide as one of its end group, was used as the silane coupler (Nanjing Xiangfei Chemical, China).

Surface modification of Al particles

The surface treatment for Al particles using the silane couplers γ-glycidoxypropyl-trimethoxysilane was carried out as follows. Ethanol aqueous solution (95 wt% concentration) and silane coupling agent (1.0 wt% of the Al mass) were added into a round-bottomed flask with reflux setting, then, the diluted hydrochloric acid was put into the mixture to adjust the ethanol aqueous solution pH to 3–5. This flask was then stirred for 20 min and the Al particles were added into the solution by ultrasonicating for 60 min. After that, the mixture was heated to 80°C and refluxed for 6 h while stirring and then cooling to room temperature, letting it set for 2 h. Finally, the products were filtered by ethanol at least three times and dried under vacuum at 110°C for 10 h.

Preparation for the Al/epoxy composites

The epoxy resin (D.E.R-331) was blended with the flexible epoxy (D.E.R-732), curing agent, Al particles, and the accelerator according to the designed mass/fraction ratio (as shown in Table 1). Then, the blend was stirred vigorously for 1 h and the obtained homogeneous mixture was degassed for about 30 min under vacuum to get rid of bubbles. After that, the liquid mixture was poured into a clean glass plate mold keeping at 60–80°C, and then was cured in an oven at 110°C for 1 h and 150°C for 5 h, respectively. Finally, the cured sample was left to cool down slowly to room temperature. Thus, the epoxy-based composites with Al powder concentrations ranging from 0 to 70 wt% were prepared.

Table 1.

Formulation of Al/epoxy composite

Raw materials	Content
Epoxy(331)/g	70
Epoxy(732)/g	30
MeHHPA/g	90
DMP-30/g	1
Al/g	Variant

Characterizations

The dielectric measurement was performed on a broadband dielectric spectrometer (Novocontrol Technology, Germany) with an Alpha-A high-performance frequency analyzer. The measurement was carried out in the frequency range of 1–10⁷ Hz at the −20 to 200°C range to investigate the dielectric property’s dependence upon temperature. The specimens for dielectric measurements were molded in the form of circular disk (diameter = 20 mm and thickness approximately 1 mm). Prior to measurement of samples, a layer of Al foil was placed on the upper and lower surfaces of the specimens.

The fractured morphology of the polymers and the dispersion of fillers in the composites were investigated using a scanning electron microscope (JSM-7000F, JEOL, Japan). The fractured surfaces were prepared in liquid nitrogen and sputtered with gold under vacuum prior to observation. The transmission electron microscopy (1200 EX, JEOL) was used to observe the alumina shell on the surface of core Al.

Tensile test of the samples was determined with a ZMGI 250 tensile tester (News SANS China), adopting standard GB/T1040-1992. Dumbbell samples were stretched at a speed of 2mm/min under a cell load of 2500N.

Results and discussion

Effect of frequencies on dielectric properties

Dielectric permittivity of a material represents the ability to store charge and reflects its dielectric properties when the material is subjected to an electric field. Figure 1(a) presents the dielectric constant of the Al/epoxy composite as a function of Al weight fraction versus the frequency at room temperature. It is obviously noted that the dielectric permittivity increases with increasing the Al content through the entire frequency range. For example, the dielectric constant of the epoxy containing 50 wt% and 70 wt% Al at 10³ Hz is 14 and 35, respectively, which is 3–8 times higher than that of the neat epoxy. The dielectric permittivity enhancement with the Al content can be explained by the MWS effect, a phenomenon that appears in heterogeneous media consisting of phases with different dielectric constant and conductivity. As shown in Figure 2, when the Al content is low, the Al particles are isolated, that is, placed so far apart that there is no interaction between them, indicating that there is insufficient accumulation of space charges in the interface between the filler and the matrix. As the Al concentration is further raised, the clusters of Al particles are formed. An increase in the dimensions of the metallic inclusion and the greater interfacial area lead to a greater average polarization, which contributes to a larger dielectric constant. The increment in dielectric constant of the epoxy with increasing filler content reveals a strong interfacial polarization at low frequencies.

Figure 1.

Dependence of dielectric properties for Al/epoxy measured on frequency and filler loading: (a) dielectric permittivity, (b) dissipation factor, and (c) AC electric conductivity at room temperature. Al: aluminum; AC: alternating current.

Figure 2.

SEM microstructures of (a) pure epoxy, (b) 5 wt% Al-epoxy, (c) 10 wt% Al-epoxy, (d) 30 wt% Al-epoxy, (e) 50 wt% Al-epoxy and (f) 70 wt% Al-epoxy. SEM: scanning electron microscopic; Al: aluminum.

Interestingly, as the frequency increases, dipoles fail to follow the alternation of applied electric field, thereby resulting in lower polarization, and gradually diminished values of dielectric permittivity for the neat epoxy.²² The dielectric constant of the composites slightly decreases with increasing the frequency. The reason can be mainly attributed to the fact that the interfacial polarizations have less time to orient themselves in the direction of the alternating field with increasing frequency.

In order to show more information about the interfacial polarization of the composites, the dissipation factors are given in Figure 1(b). The dissipation factor of the samples slightly decreases with the increasing frequency followed by an increase to a certain frequency 10⁶ Hz. The first increase in the dissipation factor can be attributed to the internal frictions caused from the orientation movements of dipoles and space charges. With further increasing the frequency, dipoles and interface polarizations cannot rapidly orient themselves in the direction of the alternating field, so, the dissipation factor declines due to the reduced friction movements. The dielectric loss tends to increase when the frequency is above 1.0 kHz and a peak appears about 10⁶ Hz for the composites, suggesting that the relaxation process is mainly related to the epoxy matrix. In the entire frequency range, the dissipation factors of the composites are still at a low level (generally below 0.02). It was reported that the insulating alumina shell (as seen in Figure 3) on the surface of core Al particle plays an important role in determining the dissipation factors of the epoxy composites.¹¹ As an interlayer between the Al cores, the nanoscale alumina shell can obviously prevent them from contacting with each other and forming direct current (DC) across the composites, thus effectively reducing the dielectric loss of the composites.

Figure 3.

TEM of a core-shell–structured Al particle. TEM: transmission electron microscopy; Al: aluminum.

Figure 1(c) gives the electrical conductivity of epoxy-containing various Al weight content measured at 20°C. The alternating current (AC) conductivity increases linearly with increasing frequency for all the samples tested and neither the DC plateau nor percolation behavior is found, suggesting that the composite is a typical insulating material. This phenomenon is owing to the insulating oxide shell as an interlayer between the Al cores, which prevent them from contacting with each other and forming electric current across the composites.

Effect of temperature on dielectric properties

Figure 4(a) exhibits the dependence of dielectric permittivity on frequency for neat epoxy at a set of temperatures from −20°C to 200°C. The dielectric constant of pure epoxy shows low values and a weak frequency-dependent behavior at lower temperatures and lower frequencies, but it increases rapidly with increasing temperature and exhibits a strong frequency-dependent behavior at higher temperatures. At the glass transition temperature (T_g), an abrupt increase in the dielectric constant is observed. For example, at 1 Hz the dielectric constant of pure epoxy rises by two orders of magnitude with the increase in temperature from 60°C to 180°C. The remarkable increase in dielectric constant of pure epoxy with temperature is associated with the enhanced interfacial polarization. Theoretically, this feature should be attributed to the glass transition movement, which often occurs in polymer materials when chain segments start to move and interact with each other. In general, below T_g, dielectric constant is not observed to change much with the external electric field due to the frozen movements of chain segments in glassy state at these temperatures, so, the contribution to dielectric constant only comes from unmovable dipoles in the polymer structure. At T_g, the free volume in epoxy is increased and thermal motions of dipoles are enhanced, which facilitates the rapid orientation of dipoles under applied electric field. After T_g, the further enhancement in the dielectric constant at low frequencies is mainly attributed to the MWS interfacial polarization effect because the thermal stimulation temperature produces an obvious accumulation of charge carriers inside the epoxy and eventually strengthens interfacial polarization effects, which, in turn, results in an enhanced increment in dielectric constant.³⁶

Figure 4.

Dependence of dielectric properties for neat epoxy on frequency: (a) dielectric constant, (b) dielectric loss and (c) electric conductivity at selected temperatures.

As shown in Figure 4(b), the dielectric loss increases with the increasing temperature, that is, 0.05 for 10 Hz at 60°C, while reaches 25.6 at 120°C. Another important feature of epoxy between −20°C and 200°C is that the dielectric loss peak, which is related to the local relaxation process, increases obviously around T_g. Thermally exited macromolecular dipoles follow faster alternations of the applied electric fields, shifting the loss peak frequency at higher values. The increase in peak frequency indicates a short relaxation time, which is consistent with an increase in the mobility of charge carriers discussed above. Such a dielectric loss response corresponds with the explanation for the dramatically enhanced dielectric permittivity with increasing temperature. Below T_g, relaxation motion of chain segments becomes very difficult because they are restricted or in the frozen movement, however, when the temperature rises to T_g (as seen in Figure 5), the mobility of the partial rotation of side groups is increased and the polymer chain segments are facilitated,³⁵ leading to increment internal friction associated with the dielectric loss.

Figure 5.

Dielectric spectra of the variation of dielectric loss versus temperature at different frequencies for neat epoxy.

Figure 4(c) reveals that the electric conductivity (σ) increases monotonically with frequency for epoxy resin below 60°C, which suggests a typical characteristic of insulating materials. However, it exhibits frequency-dependent conductivity in the certain frequency range above 80°C. Starting from 80°C, the electrical conductivity of epoxy resin displays a dramatic increase and a DC plateau that broadens to higher frequency with the increasing temperature. Such behavior is very similar to the insulator–conductor transition near the percolation threshold. When T > T_g, conductivity can be separated in two parts. The long-range transport of charge carriers in response to the applied electric field makes the conductivity in the low frequency. Above a certain frequency, the increase in AC conductivity with frequency can be described by power-law dependence. The plateau in low-frequency part above T_g is consistent with the DC-conduction behavior of percolative systems reported before, where DC conductivity is independent with frequency and increases with temperature. It is believed that the significantly enhanced ion mobility results in this dramatic increase in electrical conductivity at the higher temperature.

The frequency dependence of conductivity is governed by $σ = σ_{0} + A ω^{s}$ , where σ₀ is the DC conductivity of the material, A is a temperature-depending parameter and the exponent s lies between zero and unity.²³ Above T_g, the low-frequency DC conductivity in insulating materials follows an Arrhenius temperature dependence $σ_{0} \propto \exp (- E_{A} / k T)$ , where E_A and k are the activation energy for the conduction process and the Boltzmann constant, respectively.

For the composites, dielectric loss, ∊″ can be expressed as

∊ ″ = ∊_{dc} ″ + ∊_{MW} ″ + ∊_{D} ″

where, $∊_{dc} ″$ and $∊_{MW} ″$ are related to conduction and interfacial polarization, respectively, and $∊_{D} ″$ stands for the dipole loss factor. The conduction loss factor is given by

∊_{dc} ″ = \frac{σ_{dc}}{2 π f}

where $σ_{dc}$ and f represent the DC conductivity and frequency, respectively.

From equation (2) and Figure 4(c), it can be seen that there is no conduction loss factor contribution to the dielectric loss for Al/epoxy below T_g, because no DC conductivity appeared, thus leading to a very low dissipation factor. The dipole loss and interfacial friction at external electric fields are dominating at low temperatures. However, above T_g, the dielectric loss increases abruptly, implying that the conduction loss is dominant in the high-temperature range and turns into a large dielectric loss. The ionic conductivity resulting from impurities in epoxy mainly contributes to the DC conduction of charge carriers’ transport at the high temperature.

The frequency dependences of dielectric properties at various temperatures for 30 wt% and 70 wt% Al/epoxy composites are illustrated in Figure 6. At a temperature below 80°C, the dielectric constant of the composites increases steadily with temperature compared with that of pure epoxy, whereas, it dramatically soars at a temperature above 80°C. For instance, the dielectric permittivity of 30 wt% Al/epoxy increases from 10 for 20°C to 1470 for 200°C at 10 Hz. In comparison with the data shown in Figure 4(a), we can conclude that the remarkable increment in dielectric constant at low frequencies can be ascribed to the greatly enhanced interfacial polarizations between the Al particles and the matrix. In such a case, we can observe that the dielectric permittivity increased with the increment in Al content at a wide temperature range.

Figure 6.

Dependence of dielectric properties for Al/epoxy on frequency:(a) dielectric constant of 30 wt% Al/epoxy and (d) 70 wt% Al/epoxy, dielectric loss of (b) 30 wt% Al/epoxy and (e) 70 wt% Al/epoxy, electric conductivity of (c)30wt%Al/epoxy and (f)70 wt% Al/epoxy at selected temperatures. Al: aluminum.

In the frequency domain of the dielectric relaxation of epoxy-based composites, the primary α-transition relaxation and the secondary β-relaxation can usually be observed.³⁶ Here, the α-relaxation of 30 wt% and 70 wt% Al/epoxy are selected as examples that are demonstrated in Figures 6(b) and (e), respectively. It is clear that the peaks shift toward higher frequencies with temperature, which is similar to the results observed in pure epoxy. Typically, higher temperature results in faster movement of charge carriers, thus leading to decrease in the relaxation time. This thermal excitation of different bound charge carriers with the increase in temperature is the main reason for the increase in dielectric loss. In addition, the peaks are found to be asymmetric and the shifts in the position of peaks do not superimpose, which indicates a wide distribution of relaxation time. Another important feature of the composites is that the dielectric loss is decreased with increasing of Al filler content. This can be ascribed to the facts that: (1) the dielectric loss is primarily owing to the thermal activated charge carriers from epoxy matrix at low frequencies after T_g and (2) the insulating shell on the surface of core Al serves as a barrier layer to control the dielectric loss efficiently through blocking the electron transfer between adjacent metal cores. Therefore, the Al/epoxy composites exhibit a rather low dissipation factor compared with pure epoxy.

Figures 6(c) and (f) demonstrate the dependence of electric conductivity on frequency for 30 wt% and 70 wt% Al/epoxy at selected temperature, respectively. It is believed that the significantly enhanced charge carrier mobility results in this dramatic increase in electrical conductivity at a temperature greater than T_g, therefore, the DC loss and insulator–conductor transition behavior occurred. It is observed that the conductivity noticeably decreases with further increasing the Al content, suggesting that the DC loss mainly comes from the polymer matrix. Such an explanation corresponds with the dissipation factor curves of the Al/epoxy analyzed above.

The dependence of tensile strength and elongation at break of the composites on the Al filler concentration are shown in Figures 7 and 8. It is clear that the Al/epoxy composites possess desirable mechanical properties, and can satisfy the engineering application in embedded capacitors.³⁷

Figure 7.

Tensile strength of Al/epoxy composite as a function of volume fraction. Al: aluminum.

Figure 8.

Elongation at break of Al/epoxy composite as a function of volume fraction. Al: aluminum.

Conclusion

In this work, the Al/epoxy composites with k and low dielectric loss have been prepared by solution method and their dynamic thermal–dielectric properties have been investigated. The results show that the dielectric permittivity of the composites increases with Al content and reduces with frequency at room temperature. The dielectric permittivity, loss, and electric conductivity of the composites increase with the increase in temperature and exhibit an abrupt rise after T_g. The insulator–conductor transition behavior occurs at about T_g, at which polymer chain segments start to move and interact with each other. Large increase in the dissipation factor and conductivity with temperature is mainly attributed to the DC conduction of thermal-activated charge carriers resulting from epoxy resin. There is a shift in the α-relaxation peak position toward higher frequencies with an increase in temperature, which determines the thermally activated nature of the relaxation time.

Compared with neat epoxy, the epoxy composites containing 30 wt% and 70 wt% Al have much higher dielectric permittivities due to the enhanced interfacial polarizations between the Al and epoxy matrix and rather low dissipation factor owing to the insulating shell on the surface of core Al that serves as a barrier layer to control the dielectric loss efficiently through blocking the electron transfer between adjacent Al cores. The electric conductivity of the Al/epoxy composites shows an abrupt increase at around T_g, and the contribution to the AC conductivity of the composites mainly comes from the epoxy matrix at a temperature higher than T_g.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors gratefully acknowledge the financial supports from the National Science Foundation of China (no. 51577154), the Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology (JZK201301), the Scientific Research Program Funded by Shaanxi Provincial Education Commission (Program no.14JK1485), and the Foundation for Key Program of Ministry of Education, China (212175).

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