Ni/Al₂O₃/epoxy high-k composites with ultralow nickel content towards high-performance dielectric applications

Abstract

Nickel/alumina/epoxy three-phase composites consisting of nickel particles dispersed in alumina/epoxy composite matrix were prepared using a facile wet impregnation process. The influences of in situ formed alumina particles (i-Al₂O₃) and epoxy on the dielectric properties of the composites were investigated in detail. For the composites with nickel contents far below the percolation threshold, the main electrical conduction mechanism is hopping conduction, and the main loss comes from polarizations. Therefore, the formation of alumina and epoxy result in higher permittivity but high loss due to the enhanced interfacial polarization. For the composites with nickel contents below but near the percolation threshold, the main electrical conduction mechanism is metal-like conduction and the main loss comes from eddy currents under high frequency electric field. Accordingly, the i-Al₂O₃ and filled epoxy between nickel particles could bring both high permittivity and low loss. This paper experimentally shows how the characteristics of the interface between different phases influence the dielectric performances of composites. And the impregnation process is an effective strategy to obtain percolative metal/ceramic/polymer three-phase composites with ultralow metal content, high permittivity and low loss.

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Introduction

High-performance dielectric materials with high permittivity (high-k) and low loss have wide applications in microstrip antennas,¹ high-density energy storage,^2–4 electrical stress control and actuators, etc.^5–7 In order to achieve high permittivity, ferroelectrics could be a good choice. However, the intrinsic high dielectric loss and low dielectric strength of ferroelectrics are still great challenges. That is to say, it is difficult to realize simultaneous high permittivity, low loss and high dielectric strength in single phase materials. As an alternative, composite materials may be promising candidates for high-k applications. As we know, the dielectric properties of a composite will undergo significant changes near its percolation threshold,^8–11 and numerous researches have confirmed that percolative composites are promising candidates for high-k materials with outstanding dielectric properties, such as tunable high permittivity, high dielectric strength, low loss, etc. Therefore, various strategies have been developed to obtain high-k according to percolation theory in composite materials, such as ferroelectric/polymer composites,^12–14 binary or ternary metal (or carbon)/polymer composites,^15–17 metal (or carbon)/ceramic composites, etc.^18–21 In these composites, a certain volume fraction (lower than but still near the percolation threshold) of metal, carbon or ferroelectric powders should be dispersed into the matrix to achieve excellent dielectric performance. However, the percolation thresholds of the composites are usually high, leading to high leakage conductance loss, eddy current loss and dielectric loss. Inui et al.¹ experimentally confirmed that, for BiTiO₃/cellulose nanopaper composites consisting of BiTiO₃ particles dispersed in cellulose nanopaper, the permittivity can only be enhanced from 5.3 to 8.1 even if the volume fraction of the ferroelectric fillers is as high as 20.9 vol%. Therefore, silver nanowires were adopted as the fillers to achieve high permittivity and low percolation threshold in their work. However, the agglomeration of the nano-scale fillers may result in severe consequences, such as significantly enhanced leakage conductance loss, unstable dielectric performances, deteriorated dielectric strength and mechanical properties, etc. As discussed above, the realization of low percolation threshold and homogeneous distribution of fillers are the two main problems that hinder the percolative composites from being promising candidates for high performance dielectric applications. Therefore, huge efforts have been paid to the preparation of percolative composites with ultralow percolation threshold and homogeneously dispersed fillers. To lower the percolation threshold of the composite, fillers with high aspect ratio, such as metal nanowires, carbon nanotubes and carbon nanofibers, are usually chosen.^22–26 However, how to realize the homogeneous distribution of these nano-scale fillers in the matrix turns to be a great challenge. Up to now, many methods have been proposed, among which the surface modification process has been proved to be an effective way to prepare homogeneous percolative high-k composites with improved dielectric strength and dielectric loss.^27–31

Although numerous binary or ternary percolative high-k composites consisting of various surface modified fillers have been proposed in recent years, the preparation of high-k composites with ultralow percolation threshold, homogeneously distributed fillers and low loss is still found to be difficult. In our previous work, a facile wet chemical process was proposed to obtain porous metal/ceramic composites with ultralow percolation threshold and significantly enhanced permittivity. In those porous metal/ceramic composites, metal particles dispersed on the pore walls of porous ceramic, leading to the ultralow percolation threshold, hence the suppressed leakage conductance loss and eddy current loss.^10,11,32 Similarly, Ameli et al.³³ prepared a class of porous polymer nanocomposites consisting of multi-walled carbon nanotubes (MWCNT) dispersed on the pore walls of porous polymer. And it is shown that the porous MWCNT/polymer composites exhibit significantly high permittivity, ultralow percolation threshold and low dielectric loss. That is to say, the porous conductor/insulator composites have great potential for high performance dielectric applications. As discussed above, porous conductor/insulator composites could be good candidates for high-k materials. However, the porous microstructure will result in the decrease of permittivity because of the low dielectric constant of air. Moreover, the leakage conductance and eddy current loss at high frequency is high due to the fact that the breakdown strength of air is much lower than insulating ceramics or polymers. That is to say, the pores should be filled with insulating materials, and epoxy is a good candidate due to its excellent dielectric property.

In this paper, a class of porous nickel/alumina composites with ultralow nickel content are prepared via a wet chemical process described in our previous work.³² Moreover, the Al(NO₃)₃ is added into impregnation solution as the precursor of in situ formed alumina (i-Al₂O₃), which could separate the nickel particles from each other. In order to improve the dielectric property of composites, epoxy is further filled into the pores to form Ni/Al₂O₃/epoxy three phase composites.

Results and discussion

The X-ray diffraction (XRD) patterns of the Ni/Al₂O₃ composites using Ni(NO₃)₂ solution (solution A: 1 mol L⁻¹ Ni²⁺) and Ni(NO₃)₂/Al(NO₃)₃ solution (solution B: 1 mol L⁻¹ Ni²⁺ + 0.5 mol L⁻¹ Al³⁺) are shown in Fig. 1. As shown in Fig. 1a–c, the Ni/Al₂O₃(A) composites (prepared by solution A) are composed of nickel and alumina, no other additional phases are observed. As shown in Fig. 1d–f, the components of Ni/Al₂O₃(B) composites (prepared by solution B) are nickel and alumina, where the alumina consists of alumina matrix and in situ formed alumina (i-Al₂O₃). There are no differences in components between the composites prepared by two kinds of impregnation solutions. It indicated that there is no existence of Al(NO₃)₃ after calcination and reduction, and the crystal structure of i-Al₂O₃ is same as alumina matrix. Furthermore, the addition of Al(NO₃)₃ has no influence on the transformation of Ni(NO₃)₂ into Ni, and nickel oxides were totally reduced into nickel during the reduction process. As we know, high temperature (>1000 °C) may result in the reactions between nickel and alumina,^34–36 leading to the appearance of unexpected phases in the composites. In the present work, the Ni/Al₂O₃ composites were prepared via a low temperature impregnation–reduction process. Accordingly, harmful reactions between nickel and alumina could be avoided.

Fig. 1 XRD patterns of the composites with different nickel contents prepared using solution A (a–c) and solution B (d–f).

Fig. 2 shows the fractured surface scanning electron microscope (SEM) images of the composites prepared using solution A. As shown in Fig. 2a–c, nickel particles are homogeneously distributed on the pore walls of porous alumina matrix in the Ni/Al₂O₃(A) composites. And the distance between nickel particles become closer with increasing nickel content. When the nickel content reaches 2.60 vol%, some nickel particles interconnect with each other, leading to the appearance of localized nickel networks (Fig. 2c). Then, the iron/alumina interface area will be significantly enlarged due to the formation of localized nickel networks, and the composites can be regarded as a supercapacitor consisting of numerous “micro-capacitors”. As we know, when the nickel content reaches the percolation threshold, three dimensional nickel networks will be formed throughout the composite. And the percolative composites with high metal content could manifest very high dielectric permittivity due to the sharply increased amount of equivalent “micro-capacitors”. Furthermore, when the nickel content exceeds the percolation threshold, negative permittivity behavior will appear, which is very attractive for double negative metamaterials.^{10,11,37–40} However, the severe leakage conductance phenomenon and high loss are still challenges for meta-composites with negative permittivity.^8,10 As discussed above, in order to obtain high-k and low loss, the nickel contents should be near but lower than the percolation threshold. In the present work, the majority of the nickel particles are still isolated as shown in Fig. 2a–c. As shown in Fig. 2d–f, the pores of Ni/Al₂O₃(A) composites can be well filled with epoxy via the impregnation–solidification process. The XRD and SEM results indicate that the impregnation process is an effective way to prepare Ni/Al₂O₃/EP composites with homogeneous microstructures. Our recent researches show that, the morphology of nickel in the composites can be well adjusted by the impregnation conditions and hydrothermal treatment. And various nickel nanostructures, such as wires, stacked disks, porous particles, can be in situ formed on the pore walls of the porous alumina. And these unique metallic nanostructures could effectively lower the percolation threshold of composites and increase the interface area of nickel/alumina and nickel/epoxy, which are very beneficial for the reduced dielectric loss and enhanced dielectric permittivity.

Fig. 2 SEM images of Ni/Al₂O₃(A) and Ni/Al₂O₃/EP(A) composites; (a–c) are the Ni/Al₂O₃(A) composites with nickel contents of 1.0 vol%, 1.8 vol% and 2.6 vol%, respectively; (d–f) are the Ni/Al₂O₃/EP(A) composites with nickel contents of 1.0 vol%, 1.8 vol% and 2.6 vol%, respectively.

Fig. 3 shows the frequency dispersions of σ_ac for composites with different nickel loadings. We can see that, the σ_ac increases with increasing nickel content for both of the Ni/Al₂O₃ and Ni/Al₂O₃/EP composites. However, the dependences of σ_ac are different for the composites with different nickel contents. As shown in Fig. 3a, the ac conductivities σ_ac of the Ni/Al₂O₃(A) composites with nickel contents of 1.0 vol% and 1.8 vol% increase with frequency, and the σ_ac–ω relationships of the composites follow the power law σ_ac ∝ ω_n (0 < n < 1), indicating the hopping conduction behaviour.²⁴ When the nickel content reaches 2.6 vol%, the σ_ac decreases slowly with increasing frequency. As mentioned above, localized nickel networks are formed in the Ni/Al₂O₃(A) composites with nickel content of 2.6 vol% (Fig. 2c). That is to say, 2.6 vol% is near the percolation threshold. Therefore, the Ni/Al₂O₃ (f_Ni = 0.026) composite shows the metal-like frequency dispersion of conductivity due to the overwhelming contribution of skin effect relative to hopping conduction behaviour at high frequencies. As is known to us, σ_ac decreases with decreasing skin depth. And the skin depth of lossy media can be expressed as δ = (2/σωμ_r)^1/2, where δ is the skin depth, σ is the DC conductivity, ω is the frequency (ω = 2πf), and μ_r is the relative permeability. Generally, σ is constant for a certain material. Meanwhile, the value of μ_r could also be regarded as a constant (μ ≈ 1.0) at high frequency due to the fact that the cut-off frequency of a magnetic material is usually below 1 GHz. Consequently, the skin depth decreases with higher frequency, leading to the decrease of ac conductivity σ_ac.

Fig. 3 Frequency dispersions of ac conductivity for the Ni/Al₂O₃(A) and Ni/Al₂O₃/EP(A) composites (a), Ni/Al₂O₃(B) and Ni/Al₂O₃/EP(B) composites (b).

Comparing the σ_ac of Ni/Al₂O₃(A) and Ni/Al₂O₃/EP(A) composites shown in Fig. 3a, it is indicated that the filling of epoxy is not an effective way to reduce the conductivity for the composites with low nickel contents (1.0 vol% and 1.8 vol%). Nevertheless, when the nickel content is 2.6 vol%, the filling of epoxy can effectively reduce the σ_ac of the composite. For the composites with low nickel contents, the distance between nickel particles is large and it is difficult to form conductive pathways under the tested ac electric field. In other words, the composites with nickel contents of 1.0 vol% and 1.8 vol% are equivalent to capacitors. Therefore, the formation of epoxy layer between nickel particles will result in the increased capacitance of the equivalent capacitors, leading to the slight increase of ac conductivity rather than suppressed conductivity. When the nickel content is high, the distance between nickel particles is small enough to form conductive pathways under the tested ac electric field, and the composite is equivalent to an inductor which manifests metal-like conduction behaviour. Accordingly, the epoxy layers between nickel particles are equivalent to resistors which could effectively reduce the conductivity. Similarly, the function of the in situ formed alumina particles (i-Al₂O₃) is the same as the epoxy layers between nickel particles. Therefore, the values of σ_ac and frequency dispersion characteristics of the composites prepared using Ni(NO₃)₂ and Ni(NO₃)₂/Al(NO₃)₃ solutions are similar when the nickel contents are 1.0 vol% and 1.8 vol% (Fig. 3a and b). And when the nickel content reaches 2.6 vol%, the σ_ac of the composites prepared using Ni(NO₃)₂ solution is much higher than the composites prepared using Ni(NO₃)₂/Al(NO₃)₃ solutions. Moreover, the σ_ac of the composites f_Ni = 0.026/EP (1 mol L⁻¹ Ni²⁺, Fig. 3a) and f_Ni = 0.026/EP (1 mol L⁻¹ ​Ni²⁺ + 0.5 mol L⁻¹ Al³⁺, Fig. 3b) are similar to each other because of the similar influences of epoxy and i-Al₂O₃ on the composites' conduction behaviours. Based on the above discussions, we can conclude that the distribution of nickel particles in the composites is tunable with composition of the impregnation solution, leading to different frequency dispersion behaviours of conductivity. Although the in situ formation of insulating layers (i-Al₂O₃ or epoxy) between nickel particles is not an effective way to reduce the high frequency ac conductivity for the composites with nickel contents far below the percolation threshold, it is an effective method for percolative composites with nickel content near the percolation threshold.

The dielectric spectra of the composites prepared using solution A are shown in Fig. 4. We can see that, the real part of permittivity ε′ increases with higher nickel content due to the increase of the interface area between nickel, alumina and epoxy (Fig. 4a). Furthermore, a significant enhancement of ε′ is observed when the nickel content increases from 1.8 vol% to 2.6 vol% for the Ni/Al₂O₃/EP(A) composites, this phenomenon could attribute to the Maxwell–Wagner–Sillars effect near percolation threshold. However, the Ni/Al₂O₃(A) with nickel content of 2.6 vol% (f_Ni = 0.026 in Fig. 4a) shows a remarkable different frequency dispersion behaviour compared with the other composites. Correspondingly, the f_Ni = 0.026 composite exhibits a significant dielectric loss as high as tan δ ≈ 100 at low frequency (Fig. 4b). Similar dielectric dispersion behaviours and high dielectric loss have been observed in other conductor/insulator composites near the percolation threshold due to the overwhelming leakage conductance phenomenon.^41,42 Generally speaking, the significant differences in conductivity between nickel (10⁵ Ω⁻¹ cm⁻¹) and alumina (10⁻¹³ Ω⁻¹ cm⁻¹) could result in the accumulation of charges on the nickel–alumina interfaces. When the composite is put into an external alternating electric field, the interfacial polarization phenomenon occurs. At low frequencies, the interfacial polarization could keep up with the change of external alternating electric field, and the charge accumulation leads to the enhancement of real permittivity ε′. However, the interfacial polarization will gradually lag behind the external electric field with increasing frequency, leading to the decrease of ε′. However, when the nickel content is near or above the percolation threshold, the composites dielectric spectra will follow the Drude model,^10,11,41,42 leading to the anomalous frequency dispersion of permittivity (Fig. 4a). Comparing the dielectric spectra of Ni/Al₂O₃(A) and Ni/Al₂O₃/EP(A) composites in Fig. 4a, a distinct enhancement of permittivity can be observed because of the filling of epoxy, and that should be attributed to the enlarged interface areas between epoxy and Ni/Al₂O₃(A) composite. This is due to the fact that the composite can be regarded as a supercapacitor consisting of numerous “micro-capacitors”. The filling of insulating epoxy can increase the amount of “micro-capacitors”, leading to the enhancement of permittivity. Especially, the Ni/Al₂O₃/EP(A) composite with nickel content of 2.6 vol% shows a much higher permittivity and lower loss (ε′_r ≈ 65, tan δ ≈ 0.1 at 100 MHz) compared with the Ni/Al₂O₃(A) composite (f_Ni = 0.026, ε′_r ≈ 18, tan δ ≈ 80 at 100 MHz). Nevertheless, the losses of f_Ni = 0.010/EP and f_Ni = 0.018/EP are higher than those of f_Ni = 0.010 and f_Ni = 0.018 (Fig. 4b). For the composites far below the percolation threshold (1.0 vol% and 1.8 vol% nickel), the main loss comes from dielectric polarization. While for the composite near the percolation threshold (2.6 vol% nickel), the main loss comes from the leakage conductance and eddy current. Consequently, the formation of epoxy layer between nickel particles could reduce the loss of the percolative nickel/alumina composite near the percolation threshold due to the suppressed leakage conductance loss. Meanwhile, the epoxy layer in the composites far below the percolation threshold will increase the composites' losses due to the enhanced interfacial polarization processes taken place on the Ni–epoxy and Al₂O₃–epoxy interfaces.

Fig. 4 Frequency dispersions of real permittivity (a) and loss (b) for the Ni/Al₂O₃(A) and Ni/Al₂O₃/EP(A) composites.

Fig. 5 shows the dielectric spectra of the Ni/Al₂O₃(B) and Ni/Al₂O₃/EP(B) composites. Similar to the composites prepared using solution A, higher nickel content and the filling of epoxy result in the increase of real dielectric permittivity (Fig. 5a). Moreover, the filling of epoxy also leads to the increase of tan δ for the composites with nickel contents of 1.0 vol% and 1.8 vol%, and the decrease of tan δ for the composite with nickel content of 2.6 vol%. Comparing Fig. 4a and 5a, we can see that the in situ formation of alumina between nickel particles can enhance the dielectric permittivity due to the enlarged Ni–Al₂O₃ and epoxy–Al₂O₃ interface areas. As shown in Fig. 4b and 5b, for the composites with nickel content of 1.0 vol% and 1.8 vol%, the i-Al₂O₃ bring in higher loss; for the composite with nickel content of 2.6 vol%, the i-Al₂O₃ could suppress the loss. When Al(NO₃)₃ added into the solution, Ni particles would be covered by i-Al₂O₃ particles after calcination. The existence of i-Al₂O₃ between Ni particles makes them isolated from each other. That is to say, the i-Al₂O₃ and epoxy play similar roles with regard to the dielectric performance of Ni/Al₂O₃/EP composites. Similarly, the enhancement of permittivity and the suppression of loss were also observed in other composites containing in situ formed insulating layers between conductive fillers.^43–45

Fig. 5 Frequency dispersion of real permittivity (a) and loss (b) for the Ni/Al₂O₃(B) and Ni/Al₂O₃/EP(B) composites.

Experimental

Preparation of the composites

The preparation of Ni/Al₂O₃/EP composites is as follows: (1) two kinds of impregnation solutions (A and B) consisting of Ni(NO₃)₂ and Al(NO₃)₃ (purity ≥ 99.8%, purchased from Sinopharm Chemical Reagent Co., Ltd.) dissolved in ethanol were prepared. In solution A, the concentration of Ni²⁺ is 1.0 mol L⁻¹, without Al³⁺ addition; in solution B, the concentration of Ni²⁺ and Al³⁺ are 1.0 mol L⁻¹ and 0.5 mol L⁻¹, respectively. Then, the porous alumina (Al₂O₃) discs with open porosity of 50% were soaked and vacuumized for 10 min in the impregnation solution. After the excess solution on the surface rubbed off, the discs were embedded into Al₂O₃ powders, and dried at 343 K for 2 h and 373 K for 2 h to remove ethanol. (2) The discs were calcined at 623 K in the air for 20 min to obtain the NiO and i-Al₂O₃ (in situ formed alumina) precursors (2Ni(NO₃)₂ = 2NiO + 4NO₂↑ + O₂↑, 4Al(NO₃)₃ = 2Al₂O₃ + 12NO₂↑ + 3O₂↑). Then the NiO precursors were reduced at 873 K under the reduction atmosphere (10% H₂/90% Ar) for 3 h to get Ni/Al₂O₃ composites. The composites with Ni content of 1.0 vol%, 1.8 vol% and 2.6 vol% were prepared. (3) The Ni/Al₂O₃ composites were soaked in epoxy/polyimide mixture (V_epoxy/V_polyimide = 1 : 1, the polyimide as curing agent of epoxy) at 373 K and vacuumized for 10 min to “press” the mixture into the porous matrix. Then, the discs were taken out and the excess mixture on the surface was rubbed off. After the solidification at 373 K for 2 h, Ni/Al₂O₃/EP three-phase composites were prepared.

Characterization

The fracture surface morphologies of the composites were obtained by SU-8020 Field Emission Scanning Electron Microscope. X-ray diffraction (XRD) patterns were recorded in air and at room temperature using the Rigaku D/max-rB X-ray diffractometer with CuKα radiation. For the permittivity measurements, the discs with silver electrodes were put between the two planar electrodes of 16453A dielectric test fixture for permittivity measurements. The permittivity measurements were carried out under AC voltage 100 mV at room temperature from 10 MHz to 1 GHz using Agilent E4991A Impedance Analyzer.⁴⁶ The conductivity σ is calculated using real and imaginary permittivity (σ = 2πfε′′_rε₀).

Conclusions

We experimentally investigated the influence of in situ formed alumina and filled epoxy on the dielectric properties of the nickel/alumina composites with ultralow nickel content in this paper. It is demonstrated that for the composites with nickel content far below the percolation threshold, the in situ formed alumina and filled epoxy result in enhanced permittivity and higher loss; for the composites with nickel contents below but near the percolation threshold, the alumina and epoxy between nickel particles could bring simultaneous enhanced permittivity and lower loss. That is to say, the modified impregnation process is an effective strategy to obtain high performance high-k metal/ceramic/polymer three-phase composites with ultralow metal content. And the clarification of the relationship between the composite's microstructures and its dielectric performances will have great significance for the development of dielectric functional materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51402271, 51401028), Foundation for Outstanding Young Scientist in Shandong Province (BS2014CL003), Project Funded by China Postdoctoral Science Foundation (2014M550375, 2015T80748), Qingdao Science and Technology Plan (14-2-4-118-jch), Fundamental Research Funds for the Central Universities (201413001), Projects Funded by Shandong and Qingdao Postdoctoral Science Foundation.

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