Improve the dielectric property and breakdown strength of composites by cladding a polymer/BaTiO3 composite layer around carbon nanotubes

doi:10.1016/j.polymer.2020.122157

Polymer

Volume 188, 3 February 2020, 122157

https://doi.org/10.1016/j.polymer.2020.122157 Get rights and content

Highlights

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We offered a strategy to build a novel core@shell structure through an organic-inorganic shell.
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Barium titanate can effectively improve the dielectric properties of composites.
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Hyperbranched aromatic polyamide played multiple roles as binder, dispersant and inert barrier.
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The dielectric constant is about 110 and the dielectric loss is only 0.28 at 1 KHz.

Abstract

This work offers a strategy for combining ceramic and conductive fillers to produce composites with high dielectric properties and breakdown strength through a novel core@shell structure. First, we successfully attached barium titanate (BT) nanoparticles to the surface of multi-walled carbon nanotubes (MWCNTs) by sol-gel method. Hyperbranched aromatic polyamide (HB-Pa) was grafted from the surface of BT-attached MWCNTs via in suit polymerization to obtain HB-Pa/BT composite-coated MWCNTs (H-B@CNTs). After the hybrid powders were compounded with epoxy resin (EP), composites with high dielectric properties were prepared. The H-B@CNTs/EP composite exhibited a dielectric constant of 110, a dielectric loss of 0.28 (1 kHz) and a breakdown strength of 6.7 kV/mm at 20 wt % MWCNTs content. The results indicated that we could tailor the dielectric properties of composites by adjusting the amount of BT nanoparticles on the surface of MWCNTs. In addition, HB-Pa played multiple roles in this system: (i) preventing BT nanoparticles from falling off the surface of MWCNTs; (ii) increasing the dispersibility and interaction of fillers in the polymer matrix; (iii) preventing MWCNTs from overlapping one another to form a conductive network to effectively improve the breakdown strength of composites.

Graphical abstract

Introduction

In recent years, electronic devices have become lightweight, miniature, flexible, and possess superior dielectric properties [[1], [2], [3]]. To meet the public's ever-increasing demands, polymer-based composites with excellent dielectric properties have drawn a lot of attention owing to their wide application in the field of microelectronics. Two routes are commonly used to fabricate polymer-based composites with high dielectric properties [4]. One way is to improve the dielectric properties of composites by introducing a large number of ceramic nanoparticles, such as BaTiO₃ (BT) [3,[5], [6], [7]], TiO₂ [8,9], BaSrTiO₃ [10,11], and CaCu₃Ti₄O₁₂ (CCTO) [[12], [13], [14]], which usually have excellent dielectric properties. Whereas these composites typically require a high filler volume fraction (>50 vol%) to achieve high dielectric properties. At such high filler concentrations, there are still problems of high dielectric loss and poor mechanical properties due to nanoparticle aggregation and voids [4]. Another classic approach is the introduction of conductive fillers into the polymer matrix. These conductive fillers, including carbon nanotubes (CNTs) [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24]], graphene [[25], [26], [27], [28], [29]], carbon fiber [30,31], and metallic nanoparticles [[32], [33], [34]], etc, can dramatically increase the dielectric constant of polymer composites by maximizing their percolative behavior. An abrupt dielectric constant can be achieved when the conductive filler concentration is close to the percolation threshold [[33], [34], [35]]. However, the defect strategy is that a slight change in the content of the conductive filler near the percolation threshold can greatly alter the dielectric properties of the composites. As the filler content increases, the composites change from insulator to conductor, which can lead to increased dielectric loss and decreased breakdown strength of the composites [1].

Composites included both ceramic nanoparticles and CNTs have been extensively researched in the past to improve the dielectric properties of the composites. Some studies reported success by combining CNTs with various ceramic nanoparticles, such as BT/CNTs/PVDF [36] and CCTO/CNTs/PVDF [37]. However, these methods usually directly mix CNTs and ceramic nanoparticles into the polymer matrix and then interrupt the formation of the conductive networks by adjusting the concentration of ceramic nanoparticles. The poor dispersibility of unmodified CNTs in the polymer matrix will increase the risk of partial breakdown of the matrix. Other reported methods included the formation of core@shell structures by ceramic nanoparticles coated on CNTs [[38], [39], [40], [41]]. In general, ceramic nanoparticles are only attached to the surface of CNTs, and this type of attachment is much weaker than the force between the polymer and CNTs. During material processing, one must ensure that nanoparticles do not fall off the surface of CNTs is a difficult task. In addition, the poor compatibility of ceramic nanoparticles and CNTs in the polymer matrix can seriously affect the dielectric properties of the composite. Therefore, how to effectively combine ceramic nanoparticles and CNTs to improve the dielectric properties of composites remains a challenging problem for future research.

Usually, an effective strategy to control the dielectric loss and breakdown strength of the percolative composites by coating a polymer shell onto the conductive filler. The polymer shell can effectively improve the compatibility between fillers and the polymer matrix, thereby indirectly improving the dielectric properties of the composites. In recent years, the application of CNTs with special core@shell structure is attractive for improving the dielectric properties of polymer@CNTs composites [18]. Our previous work reported some core–shell structures of polymer@CNTs [42,43]. The main function of the polymer shell is to prevent CNTs from overlapping with one another and improve compatibility between CNTs and the polymer matrix. At the same time, the insulating polymer shell improved the breakdown strength of conducting filler/polymer composites, which is poor in generally. Zhang et al. [44] prepared tri-layered structure composites, 0.6 wt% of CNTs/cyanate ester resin-mica paper, and their breakdown strength were 2.6 times that of CNT composites. Zhao et al. [45] reported a new type of tri-layer composite (A-B-A) with greatly increased dielectric constant and breakdown strength. The aligned carbon nanotube bundle and polydopamine-coated barium titanate nanofbers as fillers for different layers. The result shown that A-B-A composite displayed the highest breakdown strength (6.09 MV/m). The problem currently exists is that the breakdown strength of the composite can only be maintained at an ultra low concentration of the conducting filler, and the breakdown strength is still a low value.

In this work, we designed a novel core@shell structured hybrid powder to further improve the dielectric properties and breakdown strength of conducting filler/polymer composites. We first successfully attached the BT nanoparticles to the surface of CNTs by sol-gel method and obtained BT-attached CNTs (B@CNTs) hybrid powder. Hyperbranched aromatic polyamide (HB-Pa) was grafted from surface of B@CNTs hybrid powder via in suit polymerization to obtain HB-Pa/BT co-coated CNTs hybrid powder (H-B@CNTs). Epoxy resin (EP) has been widely used as a matrix of composite materials as a general high-performance polymer [[46], [47], [48]]. so we choose epoxy resin to mix with H-B@CNTs to obtain the composites. In this system, HB-Pa played an important role in improving the dielectric properties of H-B@CNTs/EP. First, the BT nanoparticles as the high dielectric ceramic were firmly adhered to the surface of the CNTs by HB-Pa, which effectively prevented BT nanoparticles from falling off the surface of CNTs. Second, the coating of HB-Pa on the surface of B@CNTs effectively increased the dispersibility of fillers in the matrix. Finally, HB-Pa prevented CNTs from overlapping with one another to form a conductive network, thereby reducing the risk of breakdown (increased the breakdown strength) of the conductive filler/polymer composites. Furthermore, we can also control the dielectric properties of the composite by adjusting the amount of BT attached. This work will show a strategy to design ceramic@conducting fillers/polymer composites with excellent dielectric properties and high breakdown strength.

Section snippets

Materials

The acidified MWCNTs (diameter: 20–40 nm, length: 10–50 μm) were supplied by Beijing DK Nano Technology Company, China. Tetrabutyl titanate and barium acetate were supplied by Aladdin. Sodium dodecyl benzene sulfonate (SDBS) was supplied by Tianjin Guangfu Fine Chemical Research Institute. Acetyl acetone, N,N-dimethylacetamide, and LiCl were purchased from Beijing Chemical Works. BT nanoparticles (<3 μm, 99.5%), Polyetheramine (D230), 3,5-diaminobenzoic acid (DABA), and N-methyl-2-pyrrolidone

Results and discussion

Fig. 2(a) and (b) show the TEM images of B@CNTs. The BT nanoparticles were scattered on the surface of MWCNTs and significantly smaller than MWCNTs. Previous reports have demonstrated that inorganic ceramic particles can be easily attached to the surface of MWCNTs by the sol-gel method due to the electrostatic interaction and π–π conjugate interaction forces of MWCNTs and SDBS [41]. Fig. 2(c) and (d) present the TEM images of H@CNTs. A polymer shell was found on the MWCNT surface, and the

Conclusion

In conclusion, we successfully synthesized core@shell structure of H-B@CNTs hybrid powders by sol-gel method and in suit polymerization. the BT nanoparticles as the high dielectric ceramic were firmly adhered to the surface of the MWCNTs by HB-Pa, which effectively prevented BT nanoparticles from falling off the surface of MWCNTs. Hybrid powder-filled EP-based composites were prepared to study the dielectric properties of the H-B@CNTs/EP composites. The coating of HB-Pa on the surface of B@CNTs

Notes

The authors declare no competing financial interest.

CRediT authorship contribution statement

Yuping Liu: Conceptualization, Data curation, Writing - original draft. Jinchao Shi: Formal analysis. Peng Kang: Software. Peng Wu: Data curation. Zheng Zhou: Data curation. Guang-Xin Chen: Writing - review & editing, Funding acquisition. Qifang Li: Project administration.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work; there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Acknowledgments

The authors gratefully acknowledge the National Natural Science Foundation of China for providing financial support (No. 51573010).

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Improve the dielectric property and breakdown strength of composites by cladding a polymer/BaTiO3 composite layer around carbon nanotubes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusion

Notes

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

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Improve the dielectric property and breakdown strength of composites by cladding a polymer/BaTiO₃ composite layer around carbon nanotubes