Scientists use the “micro-crosslinking method” to create highly elastic ferroelectric materials

On August 4, the flexible magnetoelectric functional materials and device team of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, published a research article entitled Intrinsically elastic polymer ferroelectric by precise slight crosslinking in Science. In this study, the intrinsic elasticization method of ferroelectric materials is proposed, that is, the micro-crosslinking method is used to transform the ferroelectric polymer from a linear structure to a network-like structure, and the influence of structural changes on the crystallization properties of the material is reduced while achieving elasticity by precisely adjusting the crosslinking density, and pioneering the simultaneous imparting of elasticity and ferroelectricity to the same material. Based on this, an elastic ferropolymer with both elastic and ferroelectric properties and good resistance to mechanical fatigue and ferroelectric fatigue was created.

Ferroelectric materials are functional materials, usually referring to crystalline materials with spontaneous polarization in a certain temperature range and the polarization direction can be flipped or reoriented with the change of the applied electric field, and its core is spontaneous polarization. Polarization is a polarity vector, because the atomic configuration in the unit cell causes the center of gravity of positive and negative charges to shift relative in this direction, forming an electric dipole moment, so that the entire crystal is polarized in this direction, which is called the special polarity direction. This imposes a limit on the point group symmetry of the crystals, and only 10 of the 32 crystal point groups have a special polarity direction, i.e. 1 (C1), 2 (C2), m (Cs), mm2 (C2v), 4 (C4), 4 mm (C4v), 3 (C3), 3m (C3v), 6 (C6), 6 mm (C6v). Only crystals belonging to these point groups have spontaneous polarization, that is, ferroelectric materials must be crystalline materials. This particular group of crystal dots gives ferroelectric materials many properties, enabling applications in data storage and processing, sensing and energy conversion, and nonlinear optics and optoelectronics. The elastic recovery that crystals can produce when stressed is extremely small, usually less than 2%, which is the reason why traditional ferroelectric materials are mostly brittle (inorganic) or plastic (organic).

The rapid development of wearable devices, flexible electronics and intelligent sensing has put forward higher and higher requirements for the materials used, that is, the need to maintain stable performance under complex deformation. The materials used in electronic devices can be divided into conductors, semiconductors, and insulating materials according to their conductivity, and conductors and semiconductors are currently elastic. As one of the most abundant functional materials in insulation, ferroelectric materials have not yet achieved elasticity, which limits the application of ferroelectric materials in flexible electronics and other fields. The ferroelectric properties of ferroelectric materials mainly come from their crystalline region, but the crystal itself is almost inelastic, so it is difficult to balance ferroelectricity and elasticity in the same material.

There are generally three methods for elasticization of ferroelectric materials – structural engineering, blending, and intrinsic elasticity. Samples prepared by structural engineering can only be deformed within the range of pre-strain values, requiring complex manufacturing techniques and difficulty in reducing device size. In the composite materials prepared by blending inorganic ferroelectric materials and elastomers, the ferroelectric domains of inorganic ferroelectric materials are disorganized and need to be effectively polarized to show ferroelectricity. Due to the large difference between the resistivity of inorganic ferroelectricity and elastomer, the electric field is mainly applied to the elastomer with greater resistivity during the polarization process, resulting in electrical breakdown and electromechanical breakdown of the elastomer phase. Therefore, intrinsic elasticization may be the only way to elasticize ferroelectric materials. Intrinsic elasticization can promote the development of materials, so that they have the ability to prepare large-scale solutions, improve equipment density and fatigue resistance of materials, etc.

Organoferroelectric materials include organic small molecule ferroelectric materials and polymer ferroelectric materials represented by PVDF (polyvinylidene fluoride). The ferroelectric properties of ferroelectric polymers mainly come from the dipole formed by atoms or groups with large polarity differences on both sides of the molecular chain, and dipoles pointed from one side to the other. Ferroelectric polymers are characterized by high flexibility, ease of fabrication into complex shapes, mechanical robustness and polar activity. The ferroelectricity in polymers was discovered in polyvinylidene fluoride in the 70s of the 20th century, and is a platform for effective cross-coupling between electrical energy, mechanical energy and thermal energy. Therefore, ferropolymers that combine ferroelectricity and flexibility may be the best candidates for ferroelectroelasticity. In the past few years, chemical crosslinking has made significant progress in the intrinsic elasticization of conductors and semiconductors. Since strong ferroelectric response requires high crystallinity and good elastic recovery requires low crystallinity, it is difficult for traditional chemical crosslinking methods to balance ferroelectric response and elastic recovery at the same time.

To this end, the team proposed the concept of “elastic ferroelectric materials” and designed a precise “micro-crosslinking method” to establish a network structure in the ferroelectric polymer. Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), 55/45mol%) was selected as the reaction matrix material, and polyethylene oxide diamine (PEG-diamine) with soft and long chain was selected as the crosslinking agent material, and low crosslinking density (1%~2%) was used to give linear ferropolymerization material elasticity while maintaining high crystallinity. The results show that the crystalline phase of the ferroelectric thin film after crosslinking is mainly the β phase, and the crystals are uniformly dispersed in the polymer crosslinking network. When stressed, the network-like structure can evenly disperse the external force and withstand more stress, avoiding the destruction of the crystalline region. The experimental results show that the ferroelectric film still has a good ferroelectric response under 70% strain, the residual polarization is about 4.5μC/cm2 and can remain stable during the stretching process, and it has good mechanical and ferroelectric flipping fatigue resistance, which improves reliability and service life and expands the scope of use. It can be seen that the “micro-crosslinking method” is an effective method to achieve ferroelectric elasticity. This method uses simple chemical reactions to achieve a good match between ferroelectricity and elasticity, which provides a new idea for the elasticization of ferroelectric materials. In the future, the research team will expand such methods, explore the universality of micro-crosslinking for the study of material elasticity, and explore the application of the prepared elastic ferroelectric materials in wearable electronic devices, energy conversion and storage, and dielectric drives.

The research work is supported by the Lu Jiaxi International Cooperation Team Project, the National Natural Science Foundation of China, the Qianjiang Talent Program of Zhejiang Province and the Jianbing Lingyan Project of Zhejiang Province.

Xiong Rengen, an expert in ferroelectric materials and professor of Southeast University, was invited to comment on the PERSPECTIVE column of Science at the same time, believing that this is a breakthrough work, opening up a new discipline of “elastic ferroelectricity”, and looking forward to the possible application scenarios and future development directions of elastic ferroelectric materials. (Source: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences)

Figure 1. Schematic diagram of the concept and synthesis strategy of elastic ferroelectricity

Figure 2. Ferroelectric response of elastic ferroelectricity under strain. A is a fully elastic device; B and C are the strains of fully elastic devices at 0% and 70%; D is the P-E hysteresis curve under 0~70% strain at 1kHz; E is the nominal Pmax, Pr and Ec under different strains and the corrected true Pr. Experiments show that the cross-linked ferroelectric film has a stable ferroelectric response under different tensile strains.

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