Dielectric properties and mechanism of composites by superposing expanded graphite/cyanate ester layer with carbon nanotube/cyanate ester layer
Introduction
As a kind of functional materials, high dielectric constant (high-k) materials have attracted increasing attentions worldwide owing to their ability of storing electric charges and weakening the original electric field intensity [1], [2], and thus can be used to fabricate embedded capacitors, electric field stress control materials, actuators and artificial muscles, power cable terminations, and so on [3], [4], [5]. In recent years, many high-k conductor/polymer composites have been prepared, their main advantage is that only a small addition of conductors is needed, and thus the composites can almost maintain good processing characteristics and other merits of the corresponding polymer [6], [7]. However, a critical problem of this kind of composites is that they generally have very high dielectric losses, bringing huge energy wastage and low reliability in service [8]. Therefore, how to decrease the dielectric loss of traditional high-k conductor/polymer composite is a premise for practical applications.
To solve this problem, some useful methods have been proposed. One is preparing an insulating coating on the surfaces of conductors to avoid the connection of conductors, and thus reduce the dielectric loss [9]; however the dielectric constant is also generally reduced. Another method is to improve the dispersion of the conductors by introducing another kind of fillers. We found that adding expanded graphite (EG) into multi-wall carbon nanotube (MWCNT)/cyanate ester (CE) composites can increase dielectric constant and reduce dielectric loss [10], however, this positive effect appears only when the content of EGs is very low.
In recent several years, some composites with special spatial structures, such as sandwich-like, layered, or gradient structures, were prepared [11], [12], [13]. This method is based on the available materials, and thus shows great potential for actual applications. We fabricated double-layer materials based on MWCNT/CE composite and a polyethylene (PE) film [14], and found that the PE-MWCNT/CE material shows a higher dielectric constant and much lower dielectric loss than the MWCNT/CE composite with the same loading of MWCNTs. As PE has very low dielectric constant, which brings a negative influence on increasing dielectric constant, so the higher dielectric constant is originated from the configuration of PE-MWCNT/CE materials rather than the inherent property of PE. However, PE-MWCNT/CE materials also face a challenge for actual applications due to the low melting point (∼110 °C) of PE and the poor interfacial adhesion between PE and MWCNT/CE composite. In addition, it is known that the space charge polarization (SCP) plays an important contribution to the dielectric constant of a heterogeneous system [14], [15], so the space charge transfer and accumulation in the composites are the main concerns. However, little work was done on this aspect.
In this study reported herein, a new kind of two-layer composites was fabricated. One layer is MWCNT/CE composite, and the other is EG/CE composite (the content of EGs is equal to the percolation threshold of EG/CE composites). The two-layer composites are found to have higher dielectric constant and much lower dielectric loss than both MWCNT/CE and EG/CE composites. Besides, more attentions were put on studying the space charge distribution in the layered structure, some new and interesting phenomena were observed. The equivalent circuits were also setup for helping to reveal origin behind the special dielectric behavior of the two-layer composites.
Section snippets
Materials
CE used was 2,2′-bis(4-cyanatophenyl) isopropylidene, which was purchased from Jiangdu Resin Factory, China. MWCNTs (average out diameter <10 nm, length = 5–15 μm) with a purity of 95% were bought from Shenzhen Nonotech Port Company (China). Expandable graphite (80 mesh) with a purity of 95% was bought from Baixing Graphite Company, China.
Preparation of EG/CE and MWCNT/CE composites
Appropriate amounts of CE and EGs were blended at 85 °C for 1 h with vigorous stirring, followed by maintained at 135 °C for 100–160 min to get a prepolymer. The
Design of the composites with special spatial structure
It is known that the dielectric properties of an electronic conductor/polymer composite will have a sudden increase when the content of the conductors reaches the percolation threshold (pc) because of the insulator-conductor transition, or the formation of the conductive path. This means that when the content of electronic conductors approaches pc, the dielectric properties of the composite become very sensitive to the micro and macro structures as the charge transfer and accumulation are
Conclusion
Through a two-step procedure, two-layer MWCNT/CE-5.0EG/CE composites were prepared. No resin fault is observed at the interface between the two layers. Although the content of the conductors in each layer reaches the percolation threshold, the two-layer composite has remarkably lower dielectric loss and higher dielectric constant than its two layers. These interesting results can be attributed to the unique spatial structure of the two-layer composites. The interface between the two layers not
Acknowledgements
Authors thank Natural Science Foundation of China (51173123), Major Program of Natural Science Fundamental Research Project of Jiangsu Colleges and Universities (11KJA430001), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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COOH group of La-EDTA. Moreover, the structure–property relationship of the La-EDTA BF was investigated by performing tensile strength and contact angle tests of BF monofilaments. The results showed that the average BF monofilament fracture load increased from 124.03 mN to 163.29 mN after modification; further, the contact angle between BF and 2,2′-bis(4-cyanatophenyl)isopropylidene decreased at 200 °C due to chemical compositional changes on the BF surface. This study confirms the applicability of La-EDTA BF as a reinforcement for BADCy composites.