Mingcong Yang

High-temperature capacitive energy storage demands that dielectric materials maintain low electrical conduction loss and high discharged energy density under thermal extremes. The temperature capability of dielectric polymers is limited to below 200 °C, lagging behind requirements for high-power and harsh-condition electronics. Here we report a molecular topology design for dielectric polymers with mechanical bonds that overcomes this obstacle, where cyclic polyethers are threaded onto the axles of various polyimides. From density functional theory and molecular dynamics calculations, we found that the local vibrations of the encircled polymer chains were damped by the cyclic molecules through mechanical bonding, substantially inhibiting the phonon-assisted interchain charge transport that dominates conduction loss when approaching the thermal extremes. At 250 °C, we experimentally observed a d.c. electrical resistivity four orders of magnitude greater than that of commercial polyimides, with the discharged energy density reaching 4.1 J cm^-3 with 90% charge-discharge efficiency, exceeding conventional dielectric polymers and polymer composites. These findings open up opportunities for substantially promoting the temperature capability of dielectric polymers given the rich diversity of existing molecular topologies modified with mechanical bonds.

Dielectric polymers are ubiquitous as electrical insulation in electronic devices and electrical systems. Electrical degradation of dielectric polymers tends to initiate catastrophic failure of numerous devices and systems, but its detection and early warning remain challenging. Here we report a general material strategy that signals the electrical degradation of dielectric polymers by autonomously presenting a visually discernible warning in the form of a pronounced colour change. This colour change is induced by the chromogenic response of molecular indicators blended with the polymer, which are chemically activated by the oxygen radicals generated in situ during the electrical degradation of the polymer. We unveil that the structural degradation and electrical properties of the dielectric polymer are quantitatively correlated with the colour difference. Such a chromogenic process is autonomous without the need of human intervention or other external energy, thus offering the convenience to lower or even eliminate the risk of dielectric failure.

The increasing demand for polymer dielectrics in different scenarios, such as electrical insulation cable, requires elevated electrical and mechanical properties. In order to understand the relationship between mechanical properties and the structure of grafted monomers in a grafted polypropylene (PP) system, a design framework based on the transfer-learning method has been proposed for designing grafted side chain structure in PP system. 22 samples of polypropylene-grafted polymer are synthesized. Through a well-trained multi-layer perceptron model for predicting glass transition temperature (Tg), mechanical properties of the grafted PP have been well predicted by the transfer-learning method. Meanwhile, the grafting group structures are divided into nine substructures. By assembling these substructures, the chemical space is established for prediction. Moreover, candidates with satisfied deep traps and flexible mechanical properties are screened out for reference. This work turns qualitative understanding into a quantitative analysis based on mathematical models and broadens the application of polymer materials design with a small dataset.

Ceramic capacitors are ubiquitously used in high power and pulse power applications, but their low energy density, especially at high temperatures (>150 °C), limits their fields of application. One of the reasons is the low energy efficiency under high electric fields and/or at high temperatures. In this work, equimolar Sm3+ and Ti4+ cations were doped in NaNbO3 to increase relaxor characteristics and energy storage properties. The optimal recoverable energy density Wrec of 6.5 J/cm3 and energy efficiency η of 96% were attained in the ceramics with 10% (Sm, Ti) concentration (SmT10). Dense microstructure and low dielectric loss were attributed to the high energy storage performance. Impedance spectra analysis revealed that the grain boundary resistance dominates at low temperatures, while the grain resistance dominates at high temperatures. The ceramics show stable Wrec and η in a broad temperature range of −90 to 200 °C and repeated charge–discharge cycles up to 105. The comprehensive energy storage performance indicates SmT10 ceramics are among potential candidates for ceramic capacitors working at high temperatures.