Papers by Author: Hua Ke

Abstract: The present investigation reports on the preparation of BiFeO3 nanopowders by a sol-gel method using acetylacetone as a stabilizer. Single-phase BiFeO3 nanopowders without any impurity or amorphous phases were obtained when the precursor was thermal treated at temperatures as low as 400 oC for 2 h. Acetylacetone (acac) plays an important role on lowering the formation temperature of pure phase BiFeO3 nanopowders. It is found that Bi/acac molar ratio of 1/30 was favorable for a stable sol and for the lowest crystallization temperature of pure BiFeO3 nanopowders. X-ray diffraction and Fourier transform infrared spectroscopy revealed that thermally induced crystallization process of BiFeO3 nanopowders from Bi-Fe polymeric precursor. When the thermal treated temperature was below 200 oC, only amorphous phase existed. With the temperature increasing up to 300 oC, crystallized phase, carbonate, were detected. After annealed at 400 oC, Bi-Fe precursor totally changed to rhombohedral BiFeO3 nanopowders. Scanning electron microscopy characterized morphologies of BiFeO3 nanopowders calcined at 400 oC and 500 oC. The ferroelectric transition of BiFeO3 nanopowders at 827 oC has been detected by differential thermal analysis.

Abstract: Bi4-xLaxTi3O12 (BLT, x=0, 0.75, 1, 2) ceramic powders were prepared with sol-gel and solid phase reaction processes, respectively. BLT powders were synthesized after decarbonization at 300°C for 1h followed by calcination at 800°C for 2h. The ceramics synthesized with cold isostatic pressing and sintering at 1150°C exhibit a perovskite-like structure. The influence of sintering parameters and La doping content on density, microstructure and dielectric properties of the BLT were discussed. With the increase of La doping content in Bi4-xLaxTi3O12, dielectric constant increases and dielectric loss gradually decreases. The dielectric constant and dielectric loss of Bi4-xLaxTi3O12 (x=2) are 132.7 and 0.012 at 100 KHz, respectively.

Abstract: The microstructure and ferroelectric domains of SrBi2Ta1.6Nb0.4O9 ceramics were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). The X-ray diffraction patterns show that the lattice constants a and b decrease, and c increases by doping with Nb into SBT sample. Accordingly, it has large strain and lattice distortion in the lattice This suggests that the Nb atoms partially occupy the location of the Ta atoms in the lattice. From TEM observations, the grains show (008) preferred orientations in the sample, which agrees well with the XRD results. The 90° domain walls are identified by the 90° rotation relationship of the electron diffraction pattern about the [001] zone axis. The 180° domain walls and anti-phase boundaries (APBs) in Nb-doped SBT ceramics are also observed, which are irregularly shaped and highly curved. The traditional α-fringes can be found in the Nb-doped SBT ceramics, which are the evidence of large strains in the lattice.

Abstract: In this research, SrBi2+xTa2O9 (SBT) powders with different Bi content were prepared using conventional solid-state reaction method. The SrBi2+xTa2O9 ceramics (x=0 ,0.2 ,0.4 ,0.6) were fabricated by the cold isostatic compaction and sintering in the air. Microstructure and dielectric, ferroelectric properties of SBT ceramics were investigated by XRD, SEM, EDAX and dielectric tester. The effects of Bi content on dielectric, ferroelectrc properties in SBT ceramics were discussed.

Abstract: For the purpose of studying material cluster design and shape design of a certain arc-shaped thermal-protection component rationally, the ablation behavior and thermal stress distribution are studied by using the method of finite element numerical simulation. The study includes ablation tests, numerical simulation of temperature field, calculation of ablation thickness and numerical simulation of unsteady thermal stress field of the component. The simulation results are consistent with the results of ablation tests, which shows that the shape design of the arc-shaped thermal-protection component is rational and the dangerous periods of the component ablation are the time of initial heating and initial ablation boundary retreat.

Abstract: Hydroxyapatite (HA)/titanium (Ti) composites were successfully fabricated by hot isostatic pressing at 850°C. The microstructure of 7T2HB (70Ti+20HA+10Bioglass, Vol. %) composites were systematically investigated. The main constituents of the composites are hydroxyapatite and titanium. A simulated body fluid (SBF) with the same ion concentrations as those of human plasma and pH value of 7.4 is selected to evaluate the in vitro biological properties of the composites. After 2-week immersion in SBF, the bioactive apatite formed a very dense film on the surface of the composites. The dissolution of CaO and Ca-P compounds in SBF improves the growth of apatite on the surface of the composites.

Abstract: SrBi2(Ta1-xNbx)2O9(SBTN) ferroelectric ceramics with typical bismuth layered perovskite structure were synthesized with a reactive hot-press sintering process at 1000°C for 2h. The as-sintered SBTN (x=0.2, 0.4, 0.6, 0.8) materials had a maximum relative density of 95.97%. Bi-layered perovskite structure was obtained. The grain sizes increased with increasing Nb content. In 1MHz frequency, remanent polarization (Pr) of SBTN (x=0.2, 0.4, 0.6, 0.8) were 10.0, 3.0, 1.7 and 1.0, while coercive field (Ec) of SBTN (x=0.2, 0.4, 0.6, 0.8) were 50, 32, 33 and 33kV/cm, respectively.

Abstract: Thermal analysis and Raman spectra were carried out in SrBi2Ta2O9 (SBT) nanoparticles to investigate phase transitions. Two anomalies have been observed in temperature dependence of specific heat for SBT nanoparticles. Under the combination with Raman spectra, it indicates that there exists a new ferroelectric intermediate phase in the phase-transition sequence. So we can conclude that the phase-transition sequence in SBT nanoparticles should be ferroelectric-ferroelectric-paraelectric. Moreover, the size effect was discussed in consideration of inner compressive stress in nanoparticles for this special transition behavior. The calculated results show that the SBT nanoparticles keep the ferroelectricity until the particle size is decreased to 4.2 nm.

Abstract: Sr1-xBi2+yTa2O9 (x, y=0.2, 0) nanoparticles, synthesized by a sol-gel method, are investigated by X-ray diffraction and Raman spectroscopy. Large structure distortion shows the size effects in nanoparticles, which gives rise to the Raman bands broadening. The Bi content is the most important factor responsible for non-stoichiometric SrBi2Ta2O9 structure distortion and providing the larger ferroelectric spontaneous polarization and the higher Curie temperature. The stability of the crystal structure is discussed in consideration of inner compressive stress in nanoparticles.