Dielectric hysteresis behaviors of polyvinylidene fluoride-based multilayer dielectrics controlled by confined distribution of conductive particles

Abstract

Polyvinylidene fluoride (PVDF)-based multilayer dielectrics containing alternating layers of carbon black (CB) were fabricated through layer-multiplying extrusion. Benefits from the presence of PVDF between CB-containing layers, the breakdown strength and hysteresis behaviors of the multilayer specimens were investigated. The multilayered distribution of conductive particles was considered to be preferred for controlling the energy conversion and storage of dielectric materials.

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The continued prosperity of micro-electronic industries and portable smart devices creates demand for high performance polymer based dielectric capacitors.^1–7 Currently, with the pursuit of reduced size, weight, cost, improved security and stability in advanced electronic devices, future applications of materials in these newborn devices impose great challenges to polymer/conductive particles dielectric (PCD) composites. Despite the high dielectric permittivity of the PCD composite that is anticipated in terms of the Maxwell–Wagner–Sillars interfacial polarization,⁸ the enormous loss and ultra-low breakdown strength have limited its applications, for instance, dielectric hysteresis behavior, which is considered as a dominant factor in achieving high performance energy materials.⁹ Based on the fundamental knowledge of the D–E hysteresis theory, the energy density derived from the following equation is one of crucial parameters in evaluating the energy storage of dielectric materials.where, E is the applied electric field and D is the electric displacement, which is also known as charge density or polarization, D_max refers to the electric displacement at the highest applied field. For conventional PCD materials, the inevitable formation of percolation networks commonly causes ultra-low breakdown strength, so that the D–E hysteresis behaviors were rarely concerned to the authors' best knowledge.

Amounts of approaches have been developed to enhance the breakdown strength by reducing or even cutting off the direct contact of conductive components along the electric field direction. Surficial modification and completely encapsulation on conductive particles have been demonstrated as effective methods, the core–shell like morphology would barrier the direct touch of neighboring conductive components. Grafting polar groups on the particle surface and natural oxidation were both identified to be useful for preventing the undesired conductance as they provide the mediate barrier components, resulting in low dielectric loss.^10,11 Shen et al.¹² encapsulated silver particles with an organic carbonaceous shell, achieving a higher permittivity, lower loss and much higher breakdown strength than pristine composites. In addition, three phase strategies with synergistic effect of ceramic and conductive particles were widely performed by Dang and Luo et al.^13–16 to reduce the percolation threshold and enhance anti-breakdown behavior. Though outstanding progress has been made on the conventional PCD composites, the final performances are still sensitive to filler loadings, causing a great difficulty in controlling the hysteresis behaviors.

Recently, Liu et al. demonstrated that the anisotropic alignment of particles in polymer matrix would have an excellent dielectric behavior benefited from the existence of barrier polymer phase.¹⁷ The highly oriented CNTs in polymer matrix enable a multilayer capacitor-like structure, resulting in a considerable high breakdown strength and energy density. Inspired by the anisotropic alignment strategy, layer-multiplying coextrusion technology was applied to fabricate a series of multilayered PCD composites consisting of alternating neat polymer (NP) layers and polymer/conductive filler composite (PF) layers in our recent published work.^18,19 Due to the separation of insulated NP layers, conductive fillers can be effectively distributed in confined layer spaces forming an anisotropic alignment of conductive pathways. The alternating assembly of insulated and conductive layers can be regarded as a multilayer capacitor-like structure. It has been revealed that the dielectric permittivity can be controlled by changing the distance between two adjacent conductive layers which are determined by the number of layers at a given thickness.¹⁸ However, the dielectric hysteresis behaviors and potential applications in energy storage were less reported before.

In this work, the polymer material was polyvinylidene fluoride (PVDF), while carbon black (CB) particles were chosen as conductive fillers instead of other high aspect ratio ones with a higher conductivity (such as CNTs, graphene, metal fibers, etc.) to avoid the influence of the layer-multiplying process on the orientation of fillers. Conductive CB particles (15 wt%) were firstly mixed with PVDF and melt extruded from a twin-screw extruder forming PVDF/CB pellets (denotes as cPVDF), then the dried cPVDF and neat PVDF pellets were coextruded from the layer-multiplying equipment forming 4-, 16- and 64-layer PVDF/cPVDF materials. The microstructure of each specimen was examined through polarized optical microscope (POM). As shown in Fig. 1(a)–(c), numerous interfaces can be observed between bright and dark layers (corresponding to PVDF and cPVDF layers, respectively) indicating that well-defined alternating distribution of conductive particles was obtained. With increasing the number of layers, the thickness of each layer reduces proportionally while the total thickness of each specimen maintains at 1 mm (more detail processes of sample preparation and characterization are described in the ESI†).

Fig. 1 (a–c) POM images of 4-, 16- and 64-layer PVDF/cPVDF specimens; (d) dielectric permittivity and loss factor of PVDF/cPVDF multilayer dielectrics and neat PVDF as a function of frequency.

For linear dielectrics, such as polymers or polymer composites, the eqn (1) can be simplified as follow:⁹

where ε_r is the relative dielectric permittivity and ε₀ is the vacuum permittivity (8.85 × 10⁻¹² F m⁻¹). Hence, the electric displacement is primarily dependent on ε_r and E. Fig. 1(d) shows the dielectric behaviors of PVDF/cPVDF multilayer specimens as a function of frequencies, measured along the thickness direction by using an impedance analyzer. For pure PVDF, the ε_r is basically maintained at 10 in the frequency range of 10² to 10⁵ Hz, but reduces more than half when the frequency reaches 10⁷ Hz due to the dipole polarization. However, increased ε_r can be observed when the pure PVDF was combined with cPVDF forming multilayer materials. When the number of layers reaches 64, the ε_r measured at 10² Hz is 40, approximately 4 times of the neat PVDF. On the other side, the loss factor (tan δ) of the multilayer specimens is controlled at a low level by increasing the number of layers. The maximum magnitude of 64-layer specimen is lower than 0.3 at 10⁶ Hz, still comparable to pure PVDF.

Based on the Series model of parallel-plate capacitors in a static electric field,²⁰ the ε_r of a PVDF/cPVDF multilayer dielectrics can be obtained as follow,

where, ε and φ represent the relative permittivity and volume fraction of each phase, respectively. In cPVDF layers, the loading of CB particles is 15 wt%, far beyond the percolation threshold of PVDF/CB composite system (about 7 wt%). It is known that the dielectric permittivity of an ideal conductor tends to infinity.²¹ Thus, the ε_cPVDF should be much larger than ε_PVDF in this work. Besides, the thickness ratio of PVDF and cPVDF layers is close to 1 (i.e. φ_PVDF = φ_cPVDF = 1/2), so that the ε_r of the multilayer specimens should be approximately two times of ε_PVDF, which is basically consistent with the experimental result of 4-layer specimen but tends to deviate by increasing the number of layers. Actually, as a frequency-dependent electric field is applied, the voltage of each layer would be initially allocated by their capacitances and then gradually shift to be re-allocated by the conductivity of each layer.²² Hence, the alternating assembly of PVDF and cPVDF layers with different dielectric permittivity and electrical conductivity may promote the accumulation of electric charges at the layer interfaces leading to the occurrence of interfacial polarization.⁸ With increasing the number of layers, more interfaces would appear between PVDF and cPVDF layers, accompanied by a decrease in the thickness of each layer as examined through PLM observations. It is considered to play a positive role in accelerating the accumulation of electric charges at interfaces leading to a distinct enhancement in dielectric permittivity of the specimen with a large number of layers.

Although the dielectric permittivity of multilayer dielectrics shows a promising result, the breakdown strength of PCD system is even more important for energy applications according to eqn (2). In most of conventional PCD systems, the controllable electric hysteresis behavior cannot be observed due to its extremely low breakdown strength. However, as the conductive particles are distributed in the alternating confined layer spaces, the leakage current along the direction of electric field would be inhibited. In this work, the breakdown strengths of 64-layer PVDF/cPVDF specimen and the PVDF/CB conventional composite with a similar dc conductivity were measured at the same condition, respectively. Results demonstrate that benefited from the separation of well-defined insulated PVDF layers, the breakdown strength of the multilayer specimen reaches about 9 MV m⁻¹, much higher than that of conventional one, 0.04 MV m⁻¹. Fig. 2(a) displays the surface of 64-layer specimen as the applied voltage reaches breakdown point and a cavity can be observed under an optical microscope. It is known that when an exorbitant voltage is applied on a resistor, a large extent of exothermic process would be generated. Hence, as the conductive cPVDF layers are combined with insulated PVDF layers cutting off the conductive pathways along the direction of electric field, the temperature would abnormally rise up accompanied with increasing the applied voltage, leading to the melting and catastrophic deformation of the whole material as shown in the magnified image of Fig. 2(b).

Fig. 2 (a) Surface image of 64-layer PVDF/cPVDF specimen after the breakdown test, (b) is the magnified image.

Despite extremely high permittivity can be obtained from percolative PCD composite, the over-low breakdown strength may barrier its electric displacement behavior. Thus, few reports have been put forward to illustrate the electric hysteresis behavior of PCD systems. Differently, above results reveal that the multilayer dielectrics consisting of neat polymeric insulated layers and conductive particle-filled conductive layers provide a convenient route to control the breakdown strength by changing the number of layers, so that the electric hysteresis behaviors may be observed under a moderate electric field. Fig. 3(a) shows the D–E hysteresis loops of neat PVDF and PVDF/cPVDF multilayer dielectrics. It is worth noting that the 4-layer specimen exhibits an almost linear hysteresis behavior corresponding to large energy conversion efficiency, meanwhile its electric displacement is much higher than that of neat PVDF. This indicates that the material with a multilayered assembly may have promising applications in energy field if the linear hysteresis behavior can be maintained. However, the hysteresis loop opens up and shows a non-linear ferroelectric-like behavior accompanied with increasing the number of layers, though the electric displacement enhances gradually. The increased energy dissipation occurring in the polarization process is consistent with the tendency of the loss factor measured through impedance analyzer as shown in Fig. 1(d), which is ascribed to the movement of electric charges induced by the interfacial polarization. Furthermore, Fig. 3(b) displays the released energy density (E_d) of neat PVDF and multilayer specimens. Compared with that of neat PVDF, the E_d of 4-layer specimen rises up much faster with increasing the field strength. As the electric field reaches 8 kV cm⁻¹, the E_d approaches to about 0.12 mJ cm⁻³, three times larger than that of neat PVDF, which reveals that more energy would be stored in the multilayer dielectric. However, owning to the large energy loss, the E_d of 16- and 64-layer specimens are increased limitedly, though high electric displacement is possessed. Thus, an appropriate layer number would be preferred for attaining a large E_d and high energy conversion efficiency.

Fig. 3 (a) Dielectric electric displacement of PVDF/cPVDF multilayer specimens as a function of electric field, the applied voltage is 1.5 kV; (b) release energy density of PVDF/cPVDF multilayer specimens as a function of electric field, the values are calculated from the hysteresis loops.

In summary, the alternating assembly of neat PVDF layers and CB-filled PVDF layers promotes the accumulation of electric charges at the layer interfaces leading to a distinct increase of dielectric permittivity due to the occurrence of interfacial polarization. However, benefit from the presence of PVDF between the CB-containing layers, only slight enhancement in dielectric loss occurs and the breakdown strength of 64-layer specimen is at least two orders of magnitude higher than that of PVDF/CB conventional composite at a similar conductivity level. Thus, the electric hysteresis behaviors are observed under a moderate electric field. With increasing the number of layers, the hysteresis loop tends to open up and the calculated release energy density rises up gradually. Thus, the confined alternative alignment of conductive particles in polymer matrix can be regarded as a potential route to control the energy conversion and storage of PCD materials.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (51203097, 51227802, 51420105004, 51421061) and the State Key Laboratory of Polymer Materials Engineering (sklpme2015-3-03) for financial support of this work. We also appreciate for the help in polarization tests provided by the research group of Prof. Jianguo Zhu.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11596e

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