Scientists realize electrothermal drive ferropolymers

On May 25, 2023, Beijing time, Professor Liu Yang of Huazhong University of Science and Technology cooperated with Professor Wang Qing of Pennsylvania State University to publish a research paper “Electro-thermal actuation in percolative ferroelectric polymer nanocomposites” in the journal Nature Materials. THE TEAM PROPOSED THE DESIGN STRATEGY OF ELECTROTHERMAL DRIVE FERROELECTRIC POLYMER NANOCOMPOSITES, THROUGH THE INTERFACIAL PHASE UNDER THE ACTION OF FIELD JOULE HEATING, SO THAT THE COMPOSITE MATERIAL PRODUCES HUGE STRAIN (8.1%) UNDER LOW ELECTRIC FIELD (40 MV/m) AND EXCELLENT DRIVING PERFORMANCE (11.3 J/cm3) BEYOND THE ELASTIC ENERGY DENSITY OF HUMAN MUSCLES, THEREBY SOLVING THE CORE PROBLEM OF EXCESSIVE ELECTRIC FIELD AND LOW OUTPUT DRIVING ELASTIC ENERGY OF FERROELECTRIC POLYMERS AS ELECTRIC DRIVERS. Opens up a new path for the design and application of high-performance new flexible drive materials and devices.

Ferroelectric polymers have been widely studied in the field of flexible drive and sensing due to their advantages of fast response, large driving strain, high biocompatibility, and easy processing. The mutual transformation of different molecular conformations of ferroelectric polymers can produce huge strain (about 8%), but the realization of this molecular conformational transition often requires ultra-high electric field strength (such as 500 MV/m), which is much larger than the driving electric field of piezoelectric ceramics or single crystals (generally less than 10 MV/m), which seriously limits its application in biomedicine, flexible electronics and other fields. In addition, due to the lower Young’s modulus of polymers, their driving elastic energy density is usually more than an order of magnitude lower than that of common electrodrive materials such as piezoelectric ceramics or single crystals, which greatly limits the output force of polymer drivers. Therefore, how to achieve large strain and high elastic energy density under low electric field is the core problem in designing new ferropolymer actuators.

Figure 1: Electrothermal drive performance. a, Phase change schematic; b, c, electric field-induced deformation; d, fatigue behavior; e, performance comparison.

In view of this, inspired by the basic principle of alloy-based electrothermal actuators (using electric current to generate Joule heating to cause martensitic-austenitic structural phase transitions in shape memory alloys resulting in large strains), the team proposed a new concept of interface-regulated electrothermal drive of ferropolymer nanocomposites. Different from the traditional mechanism of driving ferroelectric materials, in the electrothermal drive, the interfacial phase undergoes a phase transition of the ferroelectric-paraelectric structure induced by Joule heat generated by the applied electric field (Figure 1a), resulting in a large strain. The main contribution of the applied electric field is to provide the internal current conduction path of the composite to generate Joule heat, which does not rely on high electric fields. Therefore, it is expected to produce excellent drive performance under low electric field, which has great application prospects in drive applications. For example, the elastic energy density generated by electrothermal drive is (11.3 J/cm3), which has obvious advantages over electrostrictive drive and piezoelectric drive (Figure 1e), and no significant fatigue behavior occurs under the action of 105 cycles of electric field (Figure 1d).

Combining dielectric temperature spectroscopy (Figure 2a) and variable temperature X-ray diffraction (Figure 2b), the research team determined that the phase transition temperature of electrothermal phase transition is about 29 °C. The presence of electric field-induced phase transitions of ferroelectric-paraelectric structures is demonstrated by subfield X-ray diffraction (Figure 2c) and infrared spectroscopy (Figure 2d). The experimental results fully confirm the existence of electrothermal phase transitions in ferroelectric polymer nanocomposites. Structural characterization methods such as nano-infrared spectroscopy (Figure 3A-3D), high-definition transmission electron microscopy (Figure 3E, 3F) and other structural characterization methods are the key to realizing electrothermal drive. Density functional theory calculations prove the stable existence of polar interface conformations. Phase field simulation (Figure 3G, 3H) confirmed that the polar seepage network was unstable under Joule heating and caused a phase transition, inducing large deformation (about 8%), which was consistent with the experimental results.

Figure 2: Experimental evidence for electrothermal phase transitions. a, dielectric temperature spectrum; b, Variable temperature X-ray diffraction; c, variable field X-ray diffraction; d, variable field infrared spectroscopy.

Figure 3: Microstructure characterization. a-d, nano-infrared spectroscopy; e,f, HD electron microscopy results; g, f, phase field simulation results.

At the same time, Nature Materials’ “Research briefing” column published “Polymer actuation using a Joule-heating-induced ferroelectric phase transition”, publicizing and positively evaluating the research results. The authors also include collaborators such as Dr. Yao Zhou, a postdoctoral fellow at the State University of Pennsylvania. The research work was supported by the National Natural Science Foundation of China and the Huazhong University of Science and Technology Talent Introduction Start-up Fund. (Source: Science Network)

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