Heterostrain-enabled ultrahigh electrostrain in lead-free piezoelectric.

Piezoelectric materials provide high strain and large driving forces in actuators and can transform electrical energy into mechanical energy. Although they were discovered over 100 years ago, scientists are still searching for alternative lead-free piezoelectrics to reduce their environmental impact. Developing high-strain piezoelectric materials has been a long-term challenge, particularly challenging for the design of high-strain polycrystalline piezoelectrics containing no toxic lead element. In this work, we report one strategy to enhance the electrostrain via designing "heterostrain" through atomic-scale defect engineering and mesoscale domain engineering. We achieve an ultrahigh electrostrain of 2.3% at high temperature (220 °C) in lead-free polycrystalline ceramics, higher than all state-of-the-art piezoelectric materials, including lead-free and lead-based ceramics and single crystals. We demonstrate practical solutions for achieving high electrostrain in low-cost environmentally piezoelectric for various applications.

A triatomic carbon and derived pentacarbides with superstrong mechanical properties.

Diamond has the largest hardness of any natural material with an experimental Vickers hardness value of 90-150 GPa. Here, we reported the stable triatomic carbon allotrope with giant hardness closing that of diamond and a family of pentacarbides with superstrong mechanical properties from the state-of-the-art theoretical calculations. The triatomic carbon allotrope can be transformed into a two-dimensional carbon monolayer at a high temperature. We predicted that the triatomic carbon allotrope holds a hardness of 113.3 GPa, showing the potential capability of cracking diamond. Substitution with Al, Fe, Ir, Os, B, N, Si, W, and O element resulted in strong pentacarbides with Young's modulus of 400-800 GPa. SiC5, BC5, IrC5, and WC5 are superhard materials with Vickers hardness over 40 GPa, of which BC5 was successfully synthesized in previous experimental reports. Our results demonstrated the potential of the present strong triatomic carbon and pentacarbides as future high-performance materials.

Interfacial Bonding and Electronic Structure between Copper Thiocyanate and Hybrid Organohalide Lead Perovskites for Photovoltaic Application.

Interface engineering of halide perovskite solar-cell device layers has been showing potential for the enhancement of efficiency and stability. The replacement of spiro-OMeTAD with cheap hole extraction layer CuSCN has emerged as one effective method for the long-term large-scale application. To clarify the interfacial mechanism between the halide perovskite layer and the CuSCN layer, first-principles calculations were performed. An interface-induced electrostatic potential increase and electron-hole excitation were observed at the interfaces. A Cu-I bond and a Pb-N bond were formed at the interface. The MAI interface exhibited a larger ionic displacement and a higher interface-induced potential than the PbI interface. Strong hybridization among Cu 4d, I 5p, N 2p, and Pb 6p orbitals at the interface was found to be the origin of the electron-hole excitation, interface reconstruction, electron charge redistribution, and stability enhancement.

Graphene-like monolayer monoxides and monochlorides.

Two-dimensional monolayer materials, with thicknesses of up to several atoms, can be obtained from almost every layer-structured material. It is believed that the catalogs of known 2D materials are almost complete, with fewer new graphene-like materials being discovered. Here, we report 2D graphene-like monolayers from monoxides such as BeO, MgO, CaO, SrO, BaO, and rock-salt structured monochlorides such as LiCl, and NaCl using first-principle calculations. Two-dimensional materials containing d-orbital atoms such as HfO, CdO, and AgCl are predicted. Adopting the same strategy, 2D graphene-like monolayers from mononitrides such as scandium nitride (ScN) and monoselenides such as cadmium selenide (CdSe) are discovered. Stress engineering is found to help stabilize 2D monolayers, through canceling the imaginary frequency of phonon dispersion relation. These 2D monolayers show high dynamic, thermal, kinetic, and mechanic stabilities due to atomic hybridization, and electronic delocalization.

Enhanced Energy-Storage Density and High Efficiency of Lead-Free CaTiO3-BiScO3 Linear Dielectric Ceramics.

A novel lead-free (1 - x)CaTiO3-xBiScO3 linear dielectric ceramic with enhanced energy-storage density was fabricated. With the composition of BiScO3 increasing, the dielectric constant of (1 - x)CaTiO3-xBiScO3 ceramics first increased and then decreased after the composition x > 0.1, while the dielectric loss decreased first and increased. For the composition x = 0.1, the polarization was increased into 12.36 µC/cm2, 4.6 times higher than that of the pure CaTiO3. The energy density of 0.9CaTiO3-0.1BiScO3 ceramic was 1.55 J/cm3 with the energy-storage efficiency of 90.4% at the breakdown strength of 270 kV/cm, and the power density was 1.79 MW/cm3. Comparison with other lead-free dielectric ceramics confirmed the superior potential of CaTiO3-BiScO3 ceramics for the design of ceramics capacitors for energy-storage applications. First-principles calculations revealed that Sc subsitution of Ti-site induced the atomic displacement of Ti ions in the whole crystal lattice, and lattice expansion was caused by variation of the bond angles and lenghths. Strong hybridization between O 2p and Ti 3d was observed in both valence band and conduction band; the hybridization between O 2p and Sc 3d at high conduction band was found to enlarge the band gap, and the static dielectric tensors were increased, which was the essential for the enhancement of polarization and dielectric properties.

Thermal mismatch strain induced disorder of Y2Mo3O12 and its effect on thermal expansion of Y2Mo3O12/Al composites.

Fully dense Y2Mo3O12/Al composites were prepared by squeeze-casting. Relatively mild conditions of 750 °C/20 min/50 MPa were used in order to avoid reaction of the components. SEM, Raman spectroscopy, XRD and dilatometry were used to characterize the microstructures and morphologies of the composites. Zero thermal expansion was achieved in the temperature range where the thermal mismatch strain was zero. We show that the CTE mismatch of Al and Y2Mo3O12 results in compressive and tensile strains that distort the Y2Mo3O12 lattice. We establish a novel method to measure the negative thermal expansion (NTE) materials' CTE under strain by measuring the composites' CTE and calculating the thermal mismatch strain between the NTE ceramic and the metal matrix. The relationship between thermal strain and Raman shift is established and measured and the simulated results are in good agreement. We also find Y2Mo3O12 to have a positive CTE when the surface strain is ≥0.80 × 10-2%.

Dielectric Enhancement in Graphene/Barium Titanate Nanocomposites.

GN/BT nanocomposites were fabricated via colloidal processing methods, and ceramics were sintered through two-step sintering methods. The microstructure and morphology were characterized by X-ray diffraction, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. XRD analysis shows that all samples are perovskite phases, and the lattice parameters a and c almost decrease linearly with the increase of graphene nanosheets. The dielectric properties were tested by using precision impedance. The maximum dielectric constant at the Curie temperature for the nanocomposites with graphene addition of 3 wt % is about 16,000, almost 2 times more than that of pure BaTiO3 ceramics. The relaxation, band structure, density of states, and charge density distribution of GN/BT superlattices were calculated using first-principles calculations for the first time, and results showed the strong hybrid interactions between C 2p states and O 2p and Ti 3d orbitals.