Hong Research Group

Electromechanical Coupling Effect and Novel Physical Mechanics Behavior in Advanced Functional Materials

Advanced functional materials have promising applications in aerospace engineering, advanced manufacturing, clean energy, artificial intelligence, biomedicine and other frontier fields due to their excellent mechanical, electrical, magnetic, thermal, optical and other multi-field coupling properties. The multi-field coupling mechanics involves the interdisciplinary research of mechanics, physics and materials, and is a hot research topic in solid mechanics. The flexoelectric effect is a novel electromechanical coupling effect describing the interaction between strain gradient and polarization in dielectrics. Given that the dramatic increase of strain gradient at micro-nanoscale, the flexoelectric effect has an important influence on the multi-field coupling properties of functional materials. We are devoted to the development of first-principles calculations and experimental characterization techniques for intrinsic flexoelectricity of low dimensional materials. From the fabrication of low dimensional single crystals to the characterization techniques of flexoelectricity, to explore the unusual physical behavior induced by micro-nanoscale flexoelectricity, such as the fundamental theory and calculation method of flexoelectricity, the bending-expansion behavior and asymmetric mechanical properties of ferroelectric materials, the giant spontaneous polarization in freestanding oxide membranes, the modulation of surface/interface electron transport behavior, the novel ferroelectric domain configurations induced by flexoelectricity and other interesting phenomena of flexo-photovoltaic/magnetic/pyroelectric. This research direction provides theoretical and experimental guidance for designing novel flexoelectric MEMS components and devices.

Photovoltaic Effects in Ferroelectric and Other Perovskite Materials

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltage, coupled with reversibly switchable photovoltaic responses. In p−n junction diodes, photogenerated electron−hole (e−h) pairs are separated by built-in electric fields forming at an interface. FPV effect is completely different from the conventional p−n junction PV effect in terms of working principles. In the case of FPVs, the e−h pairs are separated by the intrinsic polarizations originating from the lack of centrosymmetry in these materials. The fundamentally different mechanism endows FPV with unique characteristics, such as switchable photovoltaic outputs, above-bandgap photovoltage, and light polarization dependence. Despite the aforementioned features, overall power conversion efficiencies (PCEs) of FPVs have remained very low due to poor photocurrents: short circuit current (Jsc) under AM1.5 solar illumination is extremely small, which is primarily due to their wide band gaps. In this regard, in order to realize the potential of FPVs, it is highly desirable to search for a novel ferroelectric material with both strong polarization and optimal band gap energy. Meanwhile, utilizing the plentiful ferroelectric domains structure and external excitations, more fascinating phenomena are expected in FPVs.

Designing Macroscale Metamaterials Inspired from Novel Physical Properties at Nanoscale

Nano-materials have novel physical and mechanical properties, such as super lubricity, super flexibility, negative thermal expansion, negative compressibility, negative Poisson's ratio effect, non-reciprocal bending properties, etc. The underlying mechanisms of these properties provide a wealth of inspirations for designing the metamaterials at macroscale. The bionic design method, which realizes structures with similar functions by imitating the structural and morphological characteristics of natural biological tissues, have become an important source of innovation for metamaterials. Here, we extend the imitations from the biological structures to the microscopic basic units such as the crystal lattices, molecules and even atoms. The novel metamaterials are designed by imitating the special physical mechanisms at micro-nanoscale, such as the damage-tolerant architected metamaterials inspired by the hardening mechanisms of polycrystalline metal and the topological mechanical metamaterials with non-reciprocal elastic response inspired by the topological atomic structure of pyrochlore crystals. We are devoted to establish an association between the physical properties at micro-nanoscale and that at macrostructures. This not only provides an innovative design method for designing new mechanical metamaterials, but also provides an effective way to expand the application of novel properties of micro-nano materials at the macroscale. Moreover, the novel mechanical metamaterials are expected to be applied in important industrial equipment and devices.

Nanoscale Construction, Characterization and Manipulation of Ferroic Functional Units by Using Scanning Probe Microscope.

This research direction is based on the atomic force microscope (AFM) system, utilizing piezoresponse force microscope (PFM), electrostatic force microscope (EFM), Kelvin probe force microscope (KPFM), conductive atomic force microscope (C-AFM), magnetic force microscope (MFM), microwave impedance microscope (sMIM) and other methods to study the ferroelectric, ferromagnetic, ferroelastic, multiferroic single crystal and ceramic materials at nanoscale. The research focuses on micro-nano scale observation and manipulation of ferroic domain structures in the presence of different excitations, to realize novel phenomena and multi-functionalization, and finally provide scientific data and potential application candidates for the future micro-nano electronics.

Lattice Dynamics and Novel Thermal Properties of Advanced Materials

Advanced materials with special heat transport properties have important applications in energy, environment, aerospace, integrated circuits and other fields. Good thermal management plays a key role in improving the stability, service life and energy utilization of components. This research is focused on ultra-low or ultrahigh thermal conductivity materials with important application prospects. By using the advanced inelastic scattering technology of neutron source or synchrotron radiation light source, and combining with first-principles simulation calculation, thermoelectric materials, ferroic materials and new phase change energy storage materials are taken as research objects. By studying the linear and nonlinear interatomic interaction and the collective vibration of atoms (phonons) in materials, the heat transport properties of materials and their possible coupling effects among force, thermal and electric fields are designed and regulated. It provides a scientific basis for the development of advanced materials with new singular heat transport operation.

High-performance Computing and Software Development

This research direction is based on the first-principles to simulate the mechanical, electrical, optical, thermal, and magnetic properties of advanced materials at the atomic scale. Computing with the task force server, Tianhe-2, and other supercomputer resources, combining with machine learning and big data mining technology, our research accelerates the development and application of new materials. Meanwhile, our research group has developed relevant high-performance computing methods, program codes, and softwares. For example, program codes for calculating material properties under an external electric field, high-efficiency and high-precision first-principles calculation methods for magnetic-mechanical coupling, etc. have been developed. Recently, the software with independent intellectual property rights for the simulation of inelastic neutron/X-ray scattering test four-dimensional dynamic structure factor software has been developed. The virtual experiment technology developed based on this software can effectively improve the test efficiency of material phonon performance and greatly simplify related data processing.