Abstract:

Objective In recent years, flexible electronic devices and smart clothing have been developed rapidly. However, traditional batteries are difficult to meet the practical needs of portability and integration of flexible products. Therefore, there is a demand on the development of flexible energy devices suitable for flexible wearable electronics. Flexoelectricity is an electromechanical coupling effect that converts mechanical energy into electrical energy through the flexural deformation of thin films, thereby powering flexible and wearable electronic devices. molybdenum disulfide (MoS₂) and polyacrylonitrile (PAN) composite films have good flexoelectric response, however, the factors affecting the flexoelectric voltage and flexoelectric coefficient are still to be studied.

Method In this study, the PAN/MoS₂ composite film will be prepared by electrospinning, and the structure, morphology and elemental composition of the composite film will be characterized by scanning electron microscope and X-ray diffractometer. The influence of different mass fractions of MoS₂ on the flexoelectric effect of PAN/MoS₂ composite films was tested through the method of cantilever beam. The flexoelectric stability of the composite films was also examined by intermittent flexoelectric testing. In addition, this paper reported a high flexoelectric voltage generated by connecting multiple composite films in series, which is applicable to micro-miniature wearable electronic devices.

Results The characterization of composite films fully demonstrated that MoS₂ was successfully loaded on the PAN fiber with good crystallinity, understanding that pure PAN film exhibits a weak flexoelectric effect. After adding MoS₂ to the film, the flexoelectric voltage and flexoelectric coefficient of the composite film were increased with the increase of mass fraction of MoS₂, and the optimum values were obtained when the mass fraction of MoS₂ was 50%, which were 0.8 V and 1.96 nC/m, respectively. Continuously increasing the content of MoS₂ in the composite film, the flexoelectric coefficient and flexoelectric voltage of the film were decreased rapidly. This is mainly because when the mass fraction of MoS₂ was high, clusters were easy to appear, and this would seriously hinder the orderly arrangement of molecular chains, thereby limiting the excursion of positive and negative electrons and inhibiting the flexoelectric effect. The flexoelectric test was carried out every other day, and the test duration was 5 000 s. During the continuous 7 d test, the flexoelectric voltage was about 0.8 V, and the consistency of the voltage value was good, indicating stable the flexoelectric effect of the PAN/MoS₂ composite film. In addition, multiple PAN/MoS₂ composite films with 50% MoS₂ mass fraction were connected in series and packaged. The flexoelectric voltage of a single composite film was about 0.8 V, and the flexoelectric voltages of 3 and 5 composite films in series were 2.3 V and 3.8 V, respectively. Connecting multiple PAN/MoS₂ composite films in series obtained high flexoelectric voltage with little voltage attenuation. Experiments proved that the flexoelectric effect obtained by mechanical flexural deformation could almost meet the needs of tiny wearable electronic devices.

Conclusion The PAN/MoS₂ composite film was prepared by electrospinning, and MoS₂ nanoparticles were successfully loaded on the surface of PAN fibers. The mass fraction of MoS₂ in PAN/MoS₂ composite film has a significant effect on its flexoelectric effect. Experiments show that when the mass fraction of MoS₂ is less than 50%, the flexoelectric voltage and flexoelectric coefficient increase with the increase of mass fraction of MoS₂, when the mass fraction of MoS₂ reaches 50%, the optimal flexoelectric voltage and flexoelectric coefficient can be obtained, and when the mass fraction of MoS₂ exceeds 50%, the MoS₂ particles will cluster and weaken the flexoelectric effect. The composite film prepared in this paper has excellent flexoelectric stability through intermittent flexoelectric test. The experimental results show that connecting the composite films in series can obtain high flexural voltage with extremely weak voltage attenuation, which can meet the needs of tiny flexible wearable electronic devices.

Key words: flexoelectric effect, MoS₂, polyacrylonitrile, electrospinning, composite film, flexoelectric coefficient, wearable electronic device