Dengwei Hu

Ferroelectric mesocrystals of bismuth sodium titanate: formation mechanism, nanostructure, and application to piezoelectric materials.

Ferroelectric mesocrystals of Bi0.5Na0.5TiO3 (BNT) with [100]-crystal-axis orientation were successfully prepared using a topotactic structural transformation process from a layered titanate H1.07Ti1.73O4·nH2O (HTO). The formation reactions of BNT mesocrystals in HTO-Bi2O3-Na2CO3 and HTO-TiO2-Bi2O3-Na2CO3 reaction systems and their nanostructures were studied by XRD, FE-SEM, TEM, SAED, and EDS, and the reaction mechanisms were given. The BNT mesocrystals are formed by a topotactic structural transformation mechanism in the HTO-Bi2O3-Na2CO3 reaction system and by a combination mechanism of the topotactic structural transformation and epitaxial crystal growth in the HTO-TiO2-Bi2O3-Na2CO3 reaction system, respectively. The BNT mesocrystals prepared by these methods are constructed from [100]-oriented BNT nanocrystals. Furthermore, these reaction systems were successfully applied to the fabrication of [100]-oriented BNT ferroelectric ceramic materials. A BNT ceramic material with a high degree of orientation, high relative density, and small grain size was achieved.

Topochemical conversion of protonated titanate single crystals into platelike Ba0.5Sr0.5TiO3 mesocrystals with controllable microstructures

This report introduces a platelike Ba0.5Sr0.5TiO3 (BST) mesocrystal synthesized by a two-step solvothermal soft chemical process. In the first step, a platelike layered titanate H1.07Ti1.73□0.27O4·xH2O (□: vacancy of Ti) (HTO) single crystal is treated in a Ba(OH)2 solution to obtain a platelike BaTiO3 (BT)/HTO/anatase nanocomposite. In the second step, the generated BT/HTO/anatase nanocomposite is treated in a Sr(OH)2 solution to obtain the platelike BST mesocrystal. The formation mechanism and nanostructure of the mesocrystal are investigated by XRD, TEM, SAED, and FESEM. The platelike BST mesocrystal is constructed from [110]-oriented BST nanocrystals and shows a set of single-crystal-like electron diffraction spots. The mesocrystal is formed via an in situ topochemical mesocrystal conversion mechanism. There is a definite relationship between the crystal-axis directions of the HTO precursor and the BST mesocrystal. The platelike BST mesocrystals with uniform microstructure and high crystallinity can be achieved by controlling the ethanol content in the reaction solvent.

Topological relations and piezoelectric responses of crystal-axis-oriented BaTiO3/CaTiO3 nanocomposites

2D crystal-axis-oriented mesocrystalline BaTiO3/CaTiO3 (BT/CT) nanocomposites with high-density heteroepitaxial interfaces were synthesized by a two-step solvothermal soft chemical process. The nanostructures, formation mechanism, topological relations between the BT and CT, and piezoelectric responses of the nanocomposites were investigated. The mesocrystalline nanocomposites are polycrystals constructed from crystal-axis-oriented BT and CT nanocrystals with the same crystal-axis orientation, respectively. The directions of the [001] and [1−10] of crystalline BT correspond to the directions of the [0−10] and [100] of crystalline CT, respectively. The mesocrystalline nanocomposites were formed via an in situ topochemical mesocrystal conversion mechanism. The density of the artificial BT/CT heteroepitaxial interface in these mesocrystalline nanocomposites can be adjusted by regulating the fraction of BT and CT in the nanocomposites. The mesocrystalline BT/CT nanocomposite with the composition close to BT/CT = 1/1 presents a large piezoelectric response owing to the lattice strain derived from its heteroepitaxial interfaces with the high density in the nanocomposite.

Structural and morphological evolution of an octahedral KNbO3 mesocrystal via self-assembly-topotactic conversion process

Different morphologies of potassium niobates were prepared via a novel two-step solvothermal process. Firstly, a precursor polyion solution of [Nb6O19]8− was prepared. Secondly, the different morphologies of potassium niobates were prepared by solvothermal treatment of the polyion solution in different water–organic solvents and adjusting the composition of the solvents. A meaningful KNbO3 (KN) mesocrystal with octahedral morphology was generated for the first time using propylamine as the solvent. The as-obtained KN mesocrystal was a polycrystal constructed from uniform [110]-crystal-axis-oriented KN nanocube crystals, which show a set of single-crystal-like electron diffraction spots. The KN mesocrystal was formed via a self-assembly-topotactic conversion mechanism including the self-assembly of plate-like particles, topotactic conversion to the KN structure and mesocrystalline crystal growth of KN nanocrystals. Such mesocrystal displays a higher piezoelectric response than common individual perovskite crystals. The success in developing the KN mesocrystal can be easily applied to the preparation of a series of piezoelectric K1−xNaxNbO3 continuous solid solution mesocrystals.