Science, Professor Lin Yuanhua and Nan Cewen

Science, Professor Lin Yuanhua and Nan Cewen, School of materials: Ultra-energy density lead free dielectric films via polymer high nanodomain design

Dielectric materials play an important role in electronic devices and power systems because of their unique ultra-high power density (ultra-fast charge-discharge rate), high voltage resistance and good reliability. However, the energy storage capacity of dielectric capacitor is relatively low, how to improve the energy density of dielectric is a long-term challenge in its continuous development. With the continuous integration and miniaturization of new electronic and electrical systems, it has become an urgent problem to improve the energy density of dielectric capacitors in the field of dielectric materials.

In September 2019, Professor Lin Yuanhua of Tsinghua University andAcademicianNan Cewen’s teamhave designed ultra-high density lead-free ferroelectric thin films by using the multi-phase nano domain design strategy. Under the guidance of the phase field simulation method, they first constructed the lead-free BiFeO3-BaTiO3-SrTiO3solid solution film, and found that the rhombohedral and rhombohedral nano domains can coexist. The minimum hysteresis loop is obtained while maintaining high polarization. The energy density of the film is up to 112J / cm3, and the energy conversion efficiency is about 80%. Relevant achievements were published in Science, a top international journal, under the title of "Ultra-energy density lead free dielectric films via polymer high nanodomain design".

High quality ferroelectric (FE) films based on PbTiO3and BaTiO3have been found to be able to withstand high voltage (1mV · cm-1) and high polarization strength, and the energy density can be increased to more than 20 J / cm3. The antiferroelectric materials (AFE), such as PbZrO3thin films, can also be used in the field of energy storage due to the high polarization caused by the antiferroelectric-ferroelectric phase transition and their low residual polarization. However, due to the weak coupling between the relaxation ferroelectric (RFE) nanodomains, the delayed polarization is more conducive to improving the energy density and showing excellent performance.

The research team first used the theoretical phase field to simulate the crystal domain structure of some specific solid solutions, and determined the material composition with the highest energy density and energy efficiency.

Based on the phase field simulation results, the team then prepared a series of (0.55-x) BFO-xBTO-0.45STO (BFBSTO, x = 0.0-0.4) films by laser pulse deposition method, and characterized them by XRD and STEM, confirmed the micro phase structure of the films, and the characterization results were highly consistent with the design simulation.

The dielectric and ferroelectric energy storage properties of BFBSTO films were further studied. These characteristics show that the BFBSTO film with multiphase nanocrystalline structure has strong relaxor ferroelectric characteristics and high polarizability, which is very beneficial to energy storage.

In addition, the team also tested the working stability and reliability of BFBSTO films (breakdown strength, leakage current, charge discharge cycle and thermal stability of energy storage). The test results show that the BFBSTO film is not only expected to be used in the field of high frequency energy storage at kilohertz and above, but also has low leakage current, excellent fatigue resistance and temperature stability, which can ensure the normal operation of the film dielectric materials under extreme conditions.

The multi-phase nanocrystalline dielectric film designed and synthesized in this experiment has excellent energy storage performance and can be used in various kinds of capacitors and thermoelectric devices. By controlling the construction of nanoscale domain structure, new ideas can be provided for the design of high-performance dielectric materials and other functional materials.

Link: DOI: 10.1126/science.aaw8109)